A lot of this is not in canon as such, but would be houseruled, so unless someone can actually cite a reference, do what works best in your saga and troupe.
1) does parma continue after a person dies?
By the RAW (p 66), Parma Magica "does not require concentration while active." That says to me that, once the ritual is performed, it is self-powered until sunrise/sunset, so, yes, it would continue. (ymmv, natch.)
2) Can you cancel your parma? I know you can concentrate to supress it, but how about cancelling it?
Since it doesn't say so, not by the RAW, no. "Supressing" and "Canceling" are markedly diff. (However, little reason that this ritual couldn't do so in your saga- not like it would radically change the game, afaik.)
3) could you make a spell to cancel parma or suppress it, if so what guidelines would it use?
PM "is a special ritual (not a ritual spell)". Nor does it state anywhere that this is a magical effect, even if it can be phrased in terms that link it mathmatically with "magic" in general.
Winds of Mundane Silence (PeVi variable) does this.
(PREVIOUS TEXT, Shown by Furion below to be in errour)
Because of what it is intended to do, I would rule "no", because the premise is safety for Hermetic magi, safety that a spell can't provide. I would compare it to Magical Might in an animal- not something that can be attacked directly with a spell, afaik (tho' with research... who knows?)
4) Does a magical item worn by a magus (a ring for instance) have to penetrate the magus' magic resistance to work?
Another tricky one. I believe that BTRAW, yes, it does- however, this often creates a major headache. I would houserule "no"- noting that such a ruling opens the door for cursed rings and such to more easily affect any mage unwise enough to put them on.