There's a lot I don't know but my general impression has been that if a diesel has good compression, messing with pistons and rings can bring grief. The logic I've heard is that most shops can't do the same quality work as the factory. In your case you're pretty sure the poor compression is not from pistons/rings.
All of what you mention can be done w/o pulling the engine ... I think.
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1986 300SDL, 362K
1984 300D, 138K
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