The fact that it can't really open its eyes and can't hold its head up would seem to indicate some type of closed head injury, or perhaps a spinal cord injury.. Something related to trauma to the central nervous system.. The labored breathing with mention of broken ribs would point me toward a collapsed or punctured lung.. Frankly, I'd say the little feller's chances aren't great..
I personally would have left it in the road and let nature take its course, or perhaps gone ahead and euthanized it myself to end it's suffering...but...that's not the choice you made. I can't fault you for that...I get it
Before I make any recommendations, let me just say that I'm NOT A VET and I've NEVER RAISED A MINK OR FERRET IN MY LIFE. Basically, I'm just spitballing here because you can't get any other help. So if you take any of my advice and he keels over dead instantaneously, well...sorry. I never said I knew what I was doing.
Ok..
If someone came to me and said "Here. Fix this." and presented me with such an animal, the only thing I can think to do would be to administer some banamine for pain and swelling, and probably also some kind of corticosteroid like dexamethasone, then just provide supportive treatment....
The ferret dosage for banamine, according to a *VERY* quick and non-thorough google search, is apparently .5-2mg/kg of bodyweight, which isn't very much AT ALL considering a mink probably doesn't weigh much more than 1kg (2.2lbs) and most injectable banamine preparations are 50mg/ml.. Sooooo, you'd be looking at .04ml to hit the maximum dosage. That's 4/100ths of one milliliter. I have teeny 1ml syringes that are broken down into 10ths, so I guess I'd be shooting for a little less than halfway to the first notch..
That would be tricky, to say the least.
As for the dexamethasone, I found one reference to an "all species" dosage for central nervous system trauma using dex that called for was 2-3mg/kg IV, tapered to 1mg/kg SQ 3-4/day.. Since I don't see myself giving a mink an IV, I'd probably just inject 1ml SQ, right up front, then drop off to maybe 1/2ml SQ, 2 or 3 times/day for...I dunno...until it either died or was well enough not to let me give it shots anymore, I reckon.
I'd probably also give it a little shot of fortified b-complex to see if that perked it up and maybe improved its appetite (though the dex will likely improve appetite, also)..
Otherwise...antibiotics, since the dex will hamper its immune system. Google says a good injectable PenG dosage for a ferret would be 1ml/5kg, so if the mink's 1kg, it would get around 2/10ths of one ML. I'd probably just shoot for somewhere less than 1/2ml and call it good, but that's just how I roll these days.
I also have bags of lactated ringer's solution on hand, and I'm guessing a ferret -- manoeverable little creatures they are -- probably have plenty of extra skin... That is to say, I'd probably just go ahead and 'saddlebag' it with SQ fluids to ensure that it's got plenty of fluids to use in processing all the meds I was putting in it.. Giving lots of meds to a dehyrated animal can be a wee bit rough on the innards..
So, that's what I'd do. I have all that stuff on hand, though.. You could probably talk your vet out of some of it in small quantities for a small fee, if you want to give it a whirl.. Banamine's not cheap, but you don't need much...so there's a few bucks. Dex is way cheap, and you wouldn't need much of that either...should be practically free. I think I paid about $15/bag for lactated ringer's from the vet, which I thought was highway robbery...when you need, though, it it's handy to have and well worth the money. You wouldn't need more than 1 bag of that, since it's a gigantic 1L bag that's probably bigger than the mink.. A disposable IV set to do the SQ ringers should be cheap, even from the vet...like, $3-5 tops.. B-complex and PenG you can probably get in a feed store for a few dollars, or if not...heck, skip the b-complex and just talk your vet out of some el-cheapo amoxicillin oral suspension and give it some of that by mouth..
Dunno what else to tell ya except...good luck! And keep us posted!
