Okay, so here's my read of the situation:
The Makarites are raiding the lowlanders. They may-- may-- decide to stop raiding them, depending. They've also got a force hot on our tail. Brushcrest probably does have an Alliance. I doubt that they are realistically going to be able to convince lowlanders being raided by the fucking Makar to give them armies to throw at an angry, "savage" mountain peoples they personally have no beef with, so Greenvalley is likely "just" facing an assault by five Brushcrest hunter pops. Throw in one or two allied units if you must.
Greenvalley has one heavy infantry unit garrisoned there. The walls only cover the main settlement; I believe, and that leaves the White Halls and a few other villages outside their protection. Our scouts will notice a Brushcrest force marching up the river, and likely the Council of Three and Speaker can convince the Antler Clans to lend one or two of their hunter pops to the defense. I say this because we have plenty of things like Obsidian Tools (1.55) that we can offer up, and the Speaker will no doubt make hay of the fact that the White Halls are in peril. In a similar line of thinking, we can probably get a rabble pop from the pilgrim village near the white halls.
So our "professional" defending units will probably be 1 heavy infantry/1-2 hunters/1 pilgrim rabble before we factor in the rabble units we can form from our population. Given that our people are literally defending the holiest site known to them, we'll likely see additional morale bonii. As a pinch move, the Speaker can accompany the defenders. It'll risk his life, but will also ensure that the men fight to the death.
So I don't think there is a realistic chance of Brushcrest "defeating" us and occupying Greenvalley-- remember, we haven't even factored in the worst case scenario where we are forced to pull back to Greenvalley's walls. So the real question is how much damage we'll do to ourselves by converting our general population into rabble units and throwing them into the meatgrinder. Ironically we've found ourselves in exactly the Makar's shoes: needing to use desperate fanatics to overcome our lack of equipment, training, and organization.
The other real danger is that of losing pops to starvation, which is exactly what will happen if our hunters don't get their asses back to Greenvalley. A huge chunk of our food production still comes from hunting. So the correct move here is to rush our force back to Greenvalley as fast as possible. The Makar will pursue, of course, but we (1) have a day's head start, (2) have dedicated light infantry scouts with excellent maneuver, and (3) remember that in the update, it stated traversing the foothills would have been so much harder without White Clansmen to guide us. We're used to the mountains, the White Clansmen are used to the mountains, and because we've traversed this tract before, we know the terrain and, worst comes to worst, we can find a place from which to ambush our pursuers. Ideally they would give up and go home before that, of course.
I think there is a strong chance that we could yet wrest utter victory from utter disaster. Brushcrest won't be guarding the rear of their forces, because they'll expect that our expeditionary force to have died to the Makarites. If we time things right, we could pull a Helm's Deep. Imagine the entire military of Brushcrest sandwiched between fanatical defenders on one end and an infuriated professional army with an elite commander on the other.
The resulting carnage will live in legend.