To my knowledge, this is already how the game works. Party members all have identical experience and levels.
No, they have seperate pools.
To be fair this suggestion is actually a technical one.
Yes, the game tries its best to keep levels the same. Usually when exp is applied to 1 character it applies it to others which is why on the front end it looks that way.
TL;DR: Characters share the XP gain, not an XP pool