Quite well summarized, Sterkarm. One additional point though: Claiming titles also applies to dukes who have vassals or in case of multi-dukes, those duke titles they have vassals in the claimable area of, vassals not being in the claimable area of any title held count as vassals of primary title.
It it intentional that you may grab a duke title that has no vassals left, meaning that if you take all the land of a duke's count vassals, you can grab and take the duke title.
Also to clarify: The Claiming Titles rule concerns forcing claims in war, not grabbing. Meaning that if you want to take a king title, you need a sound RP reason to do so. You may grab titles and usurp them, but that is just waste of prestige, unless you have RP-wise valid claim to proclaim the ruler is not a legitimate one.
Creating king titles when you have a relative of the last king in your court can be allowed, but it would require you to give the kingdom title to him right after you have created it (you still get the prestige though). If you want to have it yourself, marry a daughter of that relative you have hanging around to yourself or your heir.
In defensive crusade you may take:
A) Land held by the christian kingdom at the start of current invasion by heathens
B) Any other christian provinces held by the attacking heathens
Defensive Crusade is not launched to increase the size of your realm cheaply. The most important rule about Defensive Crusade is that all land taken from heathens will be given back to the rightful king via edit between sessions. If you want to keep that land, I'll give you 0.8 BB for each province you want to keep. If you allow it to be edited back to rightful king, I will reduce your BB by 0.2 for each province held directly by you that gets edited away.
Kingdom of Jerusalem is a special case, as if the kingdom is destroyed you may freely crusade in KoJ area (held by it at start) to re-establish it. The lands need to be given to non-heir courtiers or vassals in KoJ area afterwards though and KoJ title given to strongest vassal there. Taking back Jerusalem (province) when KoJ still exists falls under normal Defensive Crusades.
If these seem complicated, just ask for clarfication and I'll do my best to explain. The purpose of these rules is to enhance the RP aspect of the game.
And regarding Excommunication, you need solid RP reason to excommunicate people. You may not excommunicate people under 20. There is no piety requirement.
Also, you may request removal of Papal Controller by edit between sessions. I will edit highest piety guy in a NPC realm to be Papal Controller instead then.
EDIT: Added things about Excommunication, thanks Solmyr.