Even if you did a distilled water flush after the green coolant, it still seems like the grit shows up when you run the red.
I finally went the Thermocure route as well. Amazing what came out. Amazing how bad it smells too. Like a cat peed on vomit.
If you have a Scangauge - what was it telling you about the coolant temp? I have a 1970s hard-sided Apache pop-up that weighs in about 2000 pounds empty. I just towed it about 1600 miles and set the cruise on 65-70. Probably an extra 1000 pounds of people and gear on top of that. Coolant...
From the look of it, you only have two real options. Entire frame swap or try to find clean metal at some point before the cross brace where you can cut and weld.
Like previously said though - it would be hard to believe, as bad as that is, that the body isn't rusty too. Especially the...
You do not. If you don't have to pass visual inspections where you are, I would highly recommend removing the PAIR system and placing the oxygen sensors in the PAIR manifold ports. MUCH easier to service.
If you look at the exhaust manifolds, 1-3 are the front one and 4-6 are the rear one. So I think the outboard is 1-3 and the inboard is 4-6. But it doesn't really matter, since you use the same oxygen sensor for both.
Also, I am lucky enough to not have smog BUT if you have EGR issues -...
For the price, I would be very wary of "very little rust". If he sent you pictures of the engine bay and undercarriage, post those. If it has the magic dial and the correct door code, don't post any pictures, just buy it.
It is nice. The price is about $2500 high, in my opinion.
Really, you are over-complicating this. The actual damper only has two bolts. One bolt goes in the relay rod and the other goes in the bracket that mounts under the radiator. Just get under there and take it off. This is a 15 minute job, if that.