To solve the inability to evolve in your own base... just let the fact that you are on creep (that is, "powered" by an egg somewhere) override all other considerations. Then you can evolve in your base.
Then again, aliens might just build an egg in front of the human base, and then be able to evolve pretty much whereever they want to. Maybe also build in, or make it exclusively, range from the OM. Either way, I do support aliens being able to evolve in their own bases during a human attack. Eventually, if you can kill all the advanced aliens and just stand there, no one spawning in the base can really do anything if the entire human team is fully armed.