Just remember that there's no such thing as a "free mega-evo", because all mega-evolutions are "free". They happen right as the turn begins, but before any attacks fire. So, if you choose to Mega-evolve that turn, there's no way you can get hit in your normal form. You should just play Mega Gengar like you would normally on the first turn!
Well yeah.
I just mean if someone has a bad matchup at the start and wants to switch, then Gengar will be able to mega evolve without taking a hit (afterwards, but still on the 1st turn). If it becomes a bad matchup for gengar, then Gengar can switch out and remain on your team as a majorly effective trapper / revenge killer.
Pursuit and priority remain a concern though.