Say I want to powerlevel a single character. I've got four more accounts to boost with.

1. If I have the solo character tag a target, then have the target killed by the 4-man group that COULDN'T get XP from the target, will the solo character get all, some, or none of the XP?
2. If I have the solo character tag a target, then DPS to below 50%, then have the target killed by the 4-man group that COULDN'T get XP from the target, will the solo character get all, some, or none of the XP?
3. If I have the solo character tag a target, then have the target killed by the 4-man group that COULD get XP from the target, will the solo character get all, some, or none of the XP?
4. If I have the solo character tag a target, then DPS to below 50%, then have the target killed by the 4-man group that COULD get XP from the target, will the solo character get all, some, or none of the XP?

Just throwing out hypotheticals. The goal is to get a single character the most XP from using 5 accounts.