If I purchased a life insurance policy for myself and then transferred ownership to my live-in girlfriend, can this policy now be rescinded for lack of insurable interest?

An insurable interest in a person's life must exist at the time that the life insurance is purchased, not necessarily at the time of death.  So you had an insurable interest in your own life when purchased.  I would just double check your state law and read the policy to make sure.  Otherwise it reverts as per the policy probably to the estate.  Good luck.