The easiest way for most to figure out if their fish died of an M. marinum infection would be to dissect the fish and look for characteristic nodules on the internal organs. An acid-fact stain isn't going to necessarily tell you anything. You can also perform PCR-based diagnostics. However, both the staining and PCRs are only useful in diagnosing the causitive agents of the lesions themselves. If the lesions are lacking, it is unlikely the fish died of M. marinus.
As for wearing gloves, if you are paranoid about it, I suppose you might as well. But given how rare the infection is, and the fact that you are as likely to get it from a swimming pool or a lake, it is pretty unnecessary. Mycobacterial infections are also very, very slow in progression, so what you saw in those pictures took many months to occur. Given that we now all recognize the issue, if you have a persistent infection like those, inform you physician of the risks of the hobby and it will be diagnosed well before it ever gets to those stages.
T