My understanding of the Jeune Ecole is a bit different. I don't believe it was strictly a commerce-raiding strategy, but rather an application of new technologies for asymmetrical warfare. (I hate that term; all warfare is asymmetrical in some way, but that's the term we have).
Commerce raiding wasn't the only aspect, but it was a key part of the Jeune Ecole idea, which was, as you said, an asymmetrical approach. It's a major reason why their writers were also (ironically) strong advocates of ignoring traditional cruiser rules governing commerce raiding, and in favor of unrestricted attacks on commerce (because torpedo boats are even less suited to cruiser rules than submarines are). As one of the early proponents, Admiral Grivel, put it, better to target the 1000 ships of the British merchant marine instead of the 1000 guns of the British battle fleet.
They also believed that torpedo boats and submarines would be sufficiently threatening to the enemy battle line to prevent the battle line from being of great use against France, but commerce raiding was a central part of the vision of how the Jeune Ecole intended to turn their torpedo boat fleets into a war-winning weapon against the UK.
I'm basically basing my understanding of the Jeune Ecole on Roksund's book
The Jeune Ecole: the strategy of the weak.
Here's a freely available article in the US Naval War College Review that nicely summarizes the history of the Jeune Ecole and why it ended up not working out.
Can't afford to build as many battleships as Britain? When they come in for blockade, sortie squadrons of torpedo boats and submarines. Find something cheap that can deny them the use of their expensive fleet. The aim is not to wrest control of the sea from them - you cannot do that - but rather to force them to expend money, ships, crews and materiel to keep command of the sea. In short, to wage a kind of guerilla warfare.
Indeed, and a key aspect of their vision of that denial was targeting commerce in particular, as a key part of the offensive wing of that guerrilla strategy.
It was coupled with a grossly exaggerated vision of the power of torpedo boats in particular as a potential counter against battleships, but 1930s-40s submarines and aviation were both vastly more effect against battleships (as HMS Barham, HMS Prince of Wales, the Kongo and the Yamato could all attest).
The French attempted this and it had a brief shelf life. Being the French, and changing governments often, they wandered between building battleships, building cruisers for commerce raiding, and building torpedo boats and submarines. As is usual when you can't make up your mind, none of it was done very well.
Britain was able to defend against Jeune Ecole-type tactics by spending money on new technologies. Menaced by torpedo boats? Build the torpedo boat destroyer and put quick-firing 4"-6" guns on your battleships. Menaced by submarines? Build escorts and adopt convoys. Menaced by commerce-raiding cruisers? Build trade-protection cruisers and some battle-cruisers for the really sticky parts.
By the time of Tsushima in 1904, the Jeune Ecole was already over and done for, and the poor performance of French designs in the Russian fleet turned France back to building capital ships. Destroyers escorted battleships armed with layers of quick-firing guns, ships moved at high speed to avoid submarines and mine-sweepers kept sea-lanes clear of mines. Control of the sea could be very expensive, but it could be attained and kept.
The torpedo boat was basically a dead end (although one could argue that the destroyer eventually ended up fulfilling the same role as envisioned for torpedo boats, with quite a bit of success), and the fleet designed for an assymetric war against the UK was effectively useless in the eventual war against the Central Powers, where the German High Seas Fleet was hopelessly outnumbered by the Royal Navy alone, and thus left nothing for a Jeune Ecole-inspired fleet to do even if the French had kept with it.
But while many of their proposed solutions turned out to be wrong, the basic principles actually in many ways were born out by the events of the First World War. As the Jeune Ecole predicted, the massive German battlefleet was effectively useless, as it was still smaller than the Royal Navy, and thus was essentially confined to port and out of action for essentially the entire war. It provided no benefit to Germany, despite the vast expense in building it. Meanwhile, the German submarine fleet and commerce raiding did have a major effect, seriously threatening the British war effort. Despite the major investment in convoys and escorts, the U-boat threat was never completely contained during WWI (whereas the Allies did much better during WWII).
And we see a similar dynamic in WWII, where the German surface fleet is largely useless (and to the extent it does get deployed, it ends up being in commerce-raiding roles anyway, because directly confronting the British fleet is obviously suicidal), whereas the submarines do have an impact (albeit much less than in WWI, due to the improvements in technology and code-breaking). And of course, if we extend to the Pacific, conventional surface ships proved extremely vulnerable to both submarine and air attack, to the point that battleships were increasingly relegated to escort roles.
Commerce-raiding, as a war-fighting tactic, has never won a war. Even when the French went at it full-tilt in the Napoleonic wars, Britain was able to keep losses (insurance rates) to a manageable level. Raiding with surface ships is fraught with peril: you need to refuel (resupply) frequently, you need ports to send in captured ships and crews, and if you are engaged by even a very inferior warship you may take damage that imperils your mission if not your survival.
Raiding by submarines was much more effective. In WW1, it was nearly decisive, thanks to British mistakes (refusals to form convoys and to release destroyers for escorts). When the US entered the war, enough destroyers were available and - as a last, desperate measure - convoys were formed. Sinkings immediately declined - sharply - and Britain stayed in the war. In WW2, submarines were highly effective for a short time and quickly had to be withdrawn from the convoy routes because of high losses.
Taking up a purely commerce-raiding strategy is a confession that you cannot compete on the seas. In Mahanian terms, it is not control of the sea but an attempt to deny the use of the sea to the enemy. To my knowledge, there is only one case where it has succeeded: against Japan in WW2.
But 1930s-40s Germany
can't compete on the seas, no matter what strategy they adopt. The British have both better ships and more of them to start, and the expertise to build more, faster, than the Germans can. Especially as Germany is facing land-based enemies on both fronts, which means the army is going to need the bulk of their investment.
In short, their strategic situation mirrors that of post-1870s France facing a hostile UK and Germany/Italy: precisely the situation in which the Jeune Ecole arose. But unlike the Jeune Ecole's fixation with torpedo boats, we know (from the experience of World War II, especially in the Pacific, as you allowed hindsight in this hypothetical) that submarines and aircraft
can prove a serious threat to capital ships.
Germany still isn't going to be able to compete on the seas to the extent of fielding a significant battlefleet, with battleships or carriers, to the extent of being able to compete with the Royal Navy. But it
can invest heavily in land-based anti-ship aviation (both planes and training) to deny the British access to the seas around its coasts (including the English Channel after the Fall of France) and it can (once the Versailles restrictions have been abandoned) build up its submarine force for commerce raiding.
Escorts and sub-hunters can be cheaply produced and improvised from existing vessels (leftover WW1 DDs, fishing vessels), and they don't have to go hunting submarines: organized around a convoy, the submarines will either come to them or be deterred from attacking. To win, a submarine must attack and torpedo its target. To win, an escort needs only to harass the sub and prevent it from scoring.
American successes with submarine warfare in the Pacific in WW2 were in a large part due to Japan's lack of emphasis on anti-submarine warfare and generally weak industrial base. Bad American torpedoes also made them somewhat complacent; fixing those problems led to an explosion of sinkings in 1943-44, at the same time that the Japanese destroyer arm was exhausted and depleted. Merchant losses simply could not be replaced, major warships were sunk and Japan's economy collapsed.
The flipside is that the defender needs escorts for every ship/convoy that could reasonably be attacked, whereas the submarine can choose its targets. The American experience in the Pacific, despite the Japanese incompetence that allowed it, shows that it can theoretically be done, and a submarine campaign doesn't have to be nearly as crippling as that in order to have a significant marginal effect on the battlefield.
But ultimately, it comes down to the fact that the Kriegsmarine was never going to be able to challenge the RN in WWII (much less the USN when they joined), so trying to do so is a fool's errand. The best that Germany can hope for from its navy is to limit the effects of Allied naval power on the course of the war, and the best way to do that is to adopt a Jeune Ecole-style assymetric approach. Ships that are tied up on convoy duty in the Atlantic aren't doing more valuable things elsewhere, and if your aircraft can deny Allied sea access to coastal waters, that makes something like Dunkirk or (worse) Normandy much more difficult to pull off. Meanwhile, any battleships/carriers that Germany builds will end up rusting away in port somewhere, with no real effect on the war.
