I also think leaving as a merc for the final level should be further bannable because timing wise, imo, leaking everything from the get go during the final level impacts the result of the game.
Sure, leavers are taken care of by autoban. Unlike cross valuer leaving (we dont further punish for this due to the fact that going cross is a risk assumed by both duo partners), it seems more unfair for a merc builder to leave since a merc builder is a solo + considering its somewhat advantage early, its greatest disadvantage (not able to build final level) should not be avoidable.
Here is a discussion tegarding this topic in the past:
viewtopic.php?f=124&t=24768&hilit=merc+leave@matdas