(a) flap extension can do a number of things to an aircraft's handling characteristics .. so it might be difficult to come up with an all-singing, all-dancing rule of thumb for your answer .... depends on what the overall effect on a given aircraft might be .... think about drag and rolling moments (especially with prop aircraft). Keep in mind that the real Vmca at the time is very dependent upon the bank angle ..
(b) I'm not sure just what Vmca has to do with Vyse .. but it certainly is not a rule of thumb ... it is a real limitation to, and boundary for, handling under specified conditions .. and, if you fail to respect it, it might very well pick up a big bore shotgun and scatter your entrails about the countryside in the ensuing crash ....
(c) from all practical points of view, Vmca is a certification animal which, in normal operation, we just need to stay well away from ... for the bugsmashers, especially, the OEI performance achieved near Vmca does not engender a high probability of survival ... so you are treading on dangerous ground with both feet .... the risk is in ending up inverted ... or just descending into the ground under control. Neither option is attractive ..