You want a clothes iron, not a soldering iron.
Place a damp rag on top of the dent, and use the steam (extra steam button, if your iron has it), to raise the dent (hopefully - it's not 100% guaranteed).
Linseed should be mixed with mineral spirits for only the first few coats. After that, linseed should be applied without dilution.
I never had a problem with my rags overheating. I must be the only one who never had that problem, by the number of people who have warned stringently about it. But, if I were you, I'd play it safe rather than sorry.