My words literally betray me.
Does anyone know of a peaceful variant or how the pvp attacking can be removed ? Would it upset the balance of the game if you instituted a rule that players could only attack aliens ?
I doubt it would have that big of an effect. In my experience, it rarely is the strong move to attack other players. It is so much like that that I don't even think you need to disallow attacking. It will happen so little to begin with and isn't that painful otherwise.
The biggest time it can matter is late game attacks on poor players. (poor as in, no money)
Perhaps all you would need to change is assume X number of tech blockades needs to be bipassed at the end of the game?
Perhaps there can be some mechanic in game that causes blockades to be placed from attacks from the aliens?
But, making no changes and just not allowing PvP attacking would work well enough.