Don't flame me, I ain't flamin' you! Yes, it's nice and civilized here - I haven't been to that Other Place for ages...
I'm not interested in roasting anyone, just finding the best system. You may well be right that it won't be very near the intuitively-correct 50% mark, though it's probably less clear-cut for lower skills. Time permitting, I'll do some calculations on those odds (but if anyone else would care to, please feel free!).
The simpler the system, the better. (Surely as a C man you can appreciate that). One mechanic would be better than two. Would you care to tell us about the unopposed roll mechanic you use for weapons v shields?