Strife,
Our 560sl's are the same year and have most of the same problems.
I had to replace my rad because the core was weeping coolent.
I only noticed it after topping of the resivor tank twice in one month.
Only had to add about a pint each time. I thought it was my bad
radiator cap. Never experienced this in 30+ years of DIY.
Funny thing was when I removed the upper hose part of the plastic neck
stayed with the hose. It was broken at the clamp and was somehow
staying in place under pressure. I consider myself very lucky.
Not so lucky was it looked like tranny fluid was getting into the coolent.
It took a few flushes till I got it cleaned up.
Now if I can only figure out where the overflow hose goes to after it
enters the fenderwell. The overflow hose is cracked so until
I figure this out I hooked up a hose to drain to the road instead of the
engine compartment. That reminds me I need to fix that.

Your "symptoms" are what I get with the same car with a new Rad.
It is mainly because of the excess load of highway driving and the
aux fan does not kick in till 100.

Now to try and answer your questions.

1. What exactly does a rad shop use in terms of equipment or chemicals?
They would need to remove the plastic tanks do an acid bath then run a
rod/brush thru the core. Acid is not really good for the aluminum and
finding tank seals would hard.

2. Does straightening the fins help (I'd say that maybe 1-3% of my fins are bent). Is straightening risky?
1-3% is not that bad. Not really risky but try to only bend them once.

3. Do you think the shop can give me an assessment of the radiator in terms of an improvement before I try it?
None of the rad shops I know of will even try to give an assessment.

I have a question for you.
My valve covers are black. Were yours???
Yours are very bright and shinny now.
Looks great. Wish I had them.

Gary