Disengage is definitely not damned good. It isn't even good. Whether it triggers is entirely in the Imperial's control which pretty much makes it garbage right there, and the fact that it doesn't trigger during Mak's own turn usually makes the movement relatively worthless. If you are playing a mission called "Flee!" where the Imperials lose after so many turns of not wounding the Rebels, it would be an awesome ability, but as it is, it's probably his worst ability.
Execute is okay if you're using the die removal ability or you have Gideon giving you extra attacks, otherwise it will rarely be as good as Get Cocky so I've never been terribly impressed. 2 strain + positioning to get rid of the white die is usually sorta meh, but if the Imps are using Cloaking Device on their troopers it's very worthwhile as the 2 pierce won't be wasted since they're still rolling black.
Cheap Shot is probably better than Sidewinder, especially since it costs half as much. The extra wound can often mean the difference between killing a trooper or not, and having a stun for uniques or vehicles is always good. I definitely get Sidewinder, but I get Cheap Shot first.