I am not saying that the system suggested by OP is fundamentally bad, but that there is no need for it. That is not something that PP system was designed for, and there doesn’t seem to be a need for it. If anything soldiers seem to be able to do too much per turn due to willpower abuse.
More importantly, OP issue isn’t mechanic, it’s cosmetic. He did have issue with “wasted” actions in XCOMs be it reloading, taking shot, moving only one tile. OP doesn’t have a problem with wastin AP, but with how they are presented. Trying to solve a cosmetic problem by messing with mechanics seems like a silly way of going about it. Hopefully a UI artist can see his post and find a solution which will alleviate OP problem, without compromising game’s mechanics.
Your problems with [restore will] button also sounds more cosmetic. One might need to compose himself and that take time, therefore it’s an intentional action. I think turning leftover action points to restore will might be an elegant solution, though keeping it an active action helps it to stay in players mind as a valid tactical option, on par with moving, shooting or using other skills.