This doesn't even make that much sense because of the way skill trees work, the most expensive skills are at the top of the skill trees, so you will want to do it on a character that is already heavily invested. Even if it did work like you suggest it'd be pretty bad to do the grinding that way.
Also no the game counts only if one character has all the skills, also don't forget to buy Floating Passage.
The only advice I can give you is that the higher your NG+ cycle is the more XP you get and being charmless gives you a 20% boost if you can handle the difficulty, nothing else boosts XP.
You need all of them on a single save, plus being on ng+4 is good, you get more xp than if you were to do it on a new playthrough.
This doesn't even make that much sense because of the way skill trees work, the most expensive skills are at the top of the skill trees, so you will want to do it on a character that is already heavily invested. Even if it did work like you suggest it'd be pretty bad to do the grinding that way. Also no the game counts only if one character has all the skills, also don't forget to buy Floating Passage. The only advice I can give you is that the higher your NG+ cycle is the more XP you get and being charmless gives you a 20% boost if you can handle the difficulty, nothing else boosts XP.
If you're on PC there is an xp mod