Roger: I agree with your explanation and translation. I second your doubts on Claudia's translation as "good, clear German", it does sound a bit awkward.
Without more context one could think of two possible different notions expressed by 'not for me'. My first suggestion goes for "ABER das ist nichtS für mich", which is slightly less colloquial than your, btw clear-cut, expression.
Another thought leads me to a second suggestion: Since part 2 and part 3 of the sentence are linked more closely to each other, 'I am but...' explaining the '... for ME', both form an opposing statement to part 1. Yet, the point of contradiction could also be contained in 'for', so the speaker's intention could be 'the reforms you are / have been initiating aren't for me, i.e. not aiming at my (personal) welfare/advancement/... as I'm taking only a minor part in this', couldn't it?
So, what would you think of "Du besitzt eine Gabe, Reformen in Gang zu setzen, aber ich spiele dabei keine Rolle, [denn] ich bin [ja] nur der unbedeutendste Stein im Mosaik."?
At any rate, the third part of the sentence should include the 'but' Claudia omitted in her translation: "... ich bin NUR ...".