Its terrible and terribly sad to see situations like this in vehicles you like. A 170k mini suv that youre keeping for 5 years more to me would imply something you like reasonably enough to do some fixing.
The great news is that if this is in the wheelwell, and underneath coatings, there's no fine body work, so any sort of stabilizing and patching would be fine.
I think Id personally take the risk of removing some of the in-wheelwell coating, and exploring the scope of how far it goes. Remove what makes sense, grind some more, convert (with ospho) whats harder to get to but you want to reasonably stabilize. Any interior areas that will be patched over Id then get a good wax spray (Amsoil HDMP, Eastwood, etc) and coat the internal surfaces. Then coat over with an oily coating or fluid film.
Patch with reasonable thickness galvanized steel, paint both sides with a self etching primer and then black rustoleum/rust stop paint from HD, ACE, etc. Degrease your patch before painting, you want a good job on it. If you get ospho like I noted above, it can be used to help degrease and roughen the galvanized surface. Before painting, hammer it to shape, good enough is good enough here, but because of the wash, you do want to get tight seams on the edges of the patch. Id look at JB weld and stainless pop rivits, and then some JB weld over the rivit heads for extra sealing.
Then Id coat the whole area. 3M makes an underbody schutz that is probably a better bet, but the rubberized undercoating would be OK too.
Good luck, keep us posted with pics!