You probably can use helper ents to get engrs to repair fixed mg nests. the caveats (and why others don't bother) are;
1- You need a fake
func_constructible - trigger_objective_info pair for the damaged state as well as another for the repaired state. ET maps become unstable if too many entities or too many trigger_objective_infos (tois) are in the map. The former (entities) have a limit of 1024 but the game needs 100 or so during game play. The latter's limit seems to be around about 16 tois (including those already in the map) but it varies a bit.
2- Repairing mg nests are low priority and in your map minor constructs are rarely if ever built. So you would have to figure some way to script it so they would be built. I have recently experimented with priority shifting a construct by having 2 real actions (one minor, one major construct) target the same thing. Basically depending on some aiscript logic when the construct is destroyed the action block disables the alternate priority construct for a set time then swaps the two after a delay. Discussed
here it is still very experimental and would have to be expressly verified if used.
BTW there is supposed to be an ET script command that is supposed to repair the mgs. You could try having the mgs self repair after a delay.
(I don't think the MG nests have all the states of a CP. Most likely just two states (working or not) plus the invisible one when they are destroyed back to construct boxes)