From a standpoint of efficiency, and because you don't have unlimited mercs at your disposal, it's better to focus on defence and not "waste" a squad to deal with the troops at the Refugee Camp. Loot will be plenty anyway.
You can have one merc triggering the all-out attack by simply moving him to one of the last squares near the Major. It can be your weakest merc, because he won't have to fight. He'll simply retreat when he meets the defenders on the square he was moving to. The important part is that he triggers the army attack.
At that point you should have all four recruitable mercs, 15 AIM mercs and your IMP, minus the one triggering the event. So 19 defenders in total.
Now, if you bring enough explosives with you and Ivan, maybe also Barry, you can intercept all(!) 5 or 6 army squads, who start on the square right of the Wassergrab mine, for a total of 60-70 enemies. Thanks to Ivan and a bunch of explosives you can clear them all out in one single turn, greatly reducing the amount of mercs needed to defend anything.
The rest of your defending forces should split between Port Cacao mine (3 squads will hit it), Pantagruel (also 3 squads will hit it), the A2 mine (1 squad will hit it, so 1 mercs with 8 veteran milita for auto-resolve will be enough) and Fleamarket (2 or 3 squads will hit it).
Do this and you have almost nothing to retake, just a few outposts here and there, but nothing too important. All except the big fight near Wassergrab should be auto-resolvable, leaving you with tons of loot.
I found this the most efficient way of dealing with the situation, but I'm open to suggestions on how to improve!