Not exactly true - you shouldn't get paint or other coating in the area where the belt rides, but it is OK to derust the entire pulley. The belt area probably doesn't need any derusting (the belt running over that area usually abrades any rust that might form); a few licks with a wire brush ought to do the job.

Most weak acid solutions (citric, acetic, phosphoric, etc) will do the job. Some work better than others. The commercial products will usually have buffers and other agents to protect the metal.

I'd use the same chemical on everything. Just so I didn't have to handle more than one solution. Shouldn't matter all that much which one you use. YMMV

I think that is the max temp to use the rust converter. The temp after treatment shouldn't matter. But I'm not sure.

Edit: According to https://eshop.wuerth.com.mt/images/u...ets/893110.pdf , "Thermally stable up to +120°C for short periods." I take that to mean you can use it for a very short time at that temp and does not apply after treatment. You might want to verify that with another source.

Shouldn't be necessary unless you have parts near the exhaust manifold. But it doesn't do any harm.

Use Fozz for corrosion inhibitor.

Functionally everything should work correctly with out doing anything.

If you want to remove the rust for cosmetics... Well, that's going to be tricky like the others have already stated.

Here's the path I would take. New belt, upgrade sway bar w/ SPL end links, ATI Damper and remove/cleanup the idler pulleys (or get new ones depending on the miles).

Here's the rub though... I suspect you have other rust concerns in there lurking that you just haven't seen.