Because the Teutonic Order has Prussian culture, so unless Poland owns a TO core with Prussian culture, you can't make Poland release TO.
Any German culture, actually. Same group is the requirement.
You want TO as your vassal in Poland and want to hold Prussia yourself, is that it? Take the province for yourself, culture convert it to Prussian, return core to TO, force-vassalize is probably the best play from here if you really want them as your vassal and don't want to give up any of the land you already hold, but it depends on how much YOS you have and what year it is for if you can convert it before it expires. If you can't do that, you can return a couple cores you currently hold, attack and force vassalize, grant a bit of whatever land you want them to eventually hold in poland if that's what you want, release them from your subjugation, then attack them and re-force vassalize them while also taking back the original land you want to own yourself that you had returned to them (can't seize land capitals), then continue feeding them whatever it is you want them to have. Warn spam Poland and Lithuania in the truce times in between so they don't kill it. I suppose instead of releasing and force vassalizing them a second time you could force lithuania to conquer from them what of their land you want after you feed them a bit, then take it back for yourself later.
Again, nope. The rule only applies to the overlord and not to the vassal. Vassal land need to be connected.
Well you can make the claim yourself and give the claim to your vassal if he can core that province through vassal interaction.
