Sounds like you've already done most of what I suggested. Sorry I got so elementry on you back there.
It's hard to say. Personally I don't attack them if I can get it half clean, but there is really no good reason for it to be dirty at all.

As for resolving the problem at hand, I can only think to suggest that you carry the floor jack in the trunk and next time it starts grinding, lift the front wheel in the nearest parking lot and see if you can locate the source.
It is possible that the new caliper is sticking. Don't fool yourself into ruling that a new part can't be the source.

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1980 Aspen 225 super six

isn't that the case,
i did too much work in between driving my car one time and when it ran with poor acceleration i had a million variables to check. turned out one of the plug wires i put in was faulty out of the box and my no 3 cylinder wasn't firing. always go elementary first! and always be glad for difficulties. obviously your older and wiser car thinks there is something you should learn, preferably the hard way. it builds character

well, the brake thing is finally resolved. i went to ntb for new tires because they were shot, ended up getting a front alignment and new ball joints and control arm bushings and while the guy was under there he took a look, remounted the calipers and said that they had been at a slight angle and that was causing the problem.

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God took you away, then brought you back to me.