In my opinion they wouldn't be separated if she was a whitelighter. If we go by the Charmed definition of Whitelighters (I don't believe I've seen them elsewhere and I believe these were the kind suggested at the start of the thread) then they are not at all separated from humans. Perhaps it's not recommended that they co-mingle too much but they are flesh and blood beings, human in every sense except for being immortal, more than once do we see them engage in intimate relationships with other humans so had Tara returned as a whitelighter she and Willow could have resumed their relationship with no other obstacles than perhaps emotional ones.