Doesn't the "defend harvester" logic only apply to each AI protecting its own harvester? Like, if you attack AI player red's harvester, AI player blue isn't going to come charging you with everything he has. I'm also pretty sure the AI doesn't tailor its strategy to any particular map or anything...it just seems to spam defenses as part of its cookie-cutter build. I've never seen 75 civilians at once...maybe 20-30ish? And most of them aren't shockies. Anyway, best defense against these guys is attack dogs since they completely bypass all armor and hitpoints and kill them in one hit no matter what (same goes for rogue Volkovs or Molotov Brothers). The dogs are weak and die in one hit, the the civs are almost always fixated on some other target and won't shoot them until it's too late.
Also, speaking of vehicle armor, I think this prevents the medic from healing them (in response to someone's above post). I found I could get a nuke civilian from selling radar dome, but when I tried healing him repeatedly, nothing happened.