As altitude increases the minimum tolerance for altitude gross error reporting increases. Normally at RVSM altitudes somewhere around 300ft "D" is reportable as a gross hight keeping error between the capt/fo displays.
The standby as you understand is not RVSM but should be reported in the log in this case.
The 80ft between the rh and the lh side does not seem like a big deal and should be trusted as the altitude you are operating at.
1- if not in fcom doccument and will be addressed by amm limits
2- reported by primary ADC's depentant on aircraft
3- The stby is a STBY, like I said above if you lose your primary x2 you are in bad form, nobody cares about RVSM in this case, it is there only to give you a reference to terra firma!!!