It's certainly an interesting tale. I still think it's pushing it somewhat, though. Conquering a number of tiny fiefs in Provence is one thing, but that is a relatively easy accomplishment for someone in the right situation. The fiefdoms of Italy were generally weak, with tiny military capacity and little in the way of defensive alliances aside from dealings with neighbouring territories. If a minor noble who is of no real threat to any larger states meddles in their affairs, few will care. France, on the other hand, is a huge state with powerful nobles controlling lots of land rather than small fiefdoms, and with the capacity to raise an army easily of 30,000 to ward off threats, and Monaco could never match that, nor even come close. Also, the support of Grimaldi family branches would help in some respects, but the family is Italianate and it seems most all the Grimaldis were in Italy or Provence, meaning their consequence to France is nil. France was already a huge threat to this region of Italy, too, so even if a few Grimaldis were in possession of fiefs themselves which could raise troops, they would refuse to even consider the idea, since their chances of overcoming France are so tiny and the result of failure would be the forfeit of their land to the French crown and in all probability that area of Italy being overrun by the French. I don't mean to put a dampener on your ideas, but really this was an era when small states just didn't have a chance against bigger states, so they avoided conflict entirely. Indeed, for a state like Monaco you could say that this has always been the case - the size proportions are just too radically different. Monaco must be several thousand times smaller than France, after all.
Now if you wrote a TL about Monaco becoming an Italian power, a la the brief collosal rise of Milan under the Sforza (I think it was them) around 1400...now that's an intriguing, and substantially more plausible, idea...