You definitely saved it, because it looks incredible! Nice save and awesome job!
I think I know what happened with the rust wash but I'm not sure about a couple of things so I may be wrong but I'll try to help anyway.
I don't know what you used for your clear coat but you said you mixed the MIG with an orderless thinner. If both the clear coat and the thinner were enamel based, then the rust wash might have reactivated the clear coat and you were actually blending the rust into that clear coat, which is why you couldn't remove it later. The two layers became one in effect. I will usually apply pigment washes over Future/Klear acrylic clear coat as it resists thinners better. Also, I use rubbing alcohol instead of thinner because it has even less effect on base coats, acrylic or enamel.
I also think you could have used less pigments or more of the carrier, instead of having a thinned pigment to apply, it should look more like slightly tinted, or dirty water. As you know it goes on looking like one thing but dries much more pronounced and I think that had a bit to do with the final outcome.
After it dries, you should be able to brush most of it off with a large dry brush and or clean it off with a Q-tip but because the thinner reactivated the clear coat, it was stuck for good.
Having said all of that, I'm not being critical of your work as you have clearly shown you have mad skill, what's important is the end result, which is stunning. For me modeling is about speed bumps and goofs and learning from them and I have come close to ruining many finishes over time, what's important here is that you soldiered on and recovered.
Awesome work! ;D