Just a thought,

What if the part of harry's soul that is used up is actually taken by lash for two reasons;
1) she actually has been removed from his brain and knew this would happen when she protected him and (bc of newfound free will) was un willing to die. So she took the soul to use to manifest later as a teacher slash guide in later books IE GS

2) part of harry's soul had been tainted by temptation and in order to totally protect him she not only chose to remove herself from brain but also the damage to soul. This is a win win win. Harry can use soulfire instead of hellfire without taint, lash can exist as a seperate entity and JB doesn't have to worry about things getting too easy for harry with a fully cooperative lash in his mind.

Note: be gentle, I can't spell nor have I read all of the posts here so if I have stepped on any toes or repeated someone elses work or ideas I am sorry and it was not intentional