You are looking at ~ 25 bits of resolution, basically a lab grade instrument performance.
Ignoring temperature, load/line/EMI effects.
Do an error budget end to end, I think you will find doing 12 bits to 1/2 LSB over time/temp,
at DC, is a challenge. Include noise analysis.
Just the drift in passives to do the AC trim will be a killer. You might look into a simultaneous sampling
technique to get the CMR you are trying to achieve, but 1 Mhz I am thinking a staggering goal.
An AD7760 can hit 2.5 MSPS @ 24 bits, maybe a starting point. Maybe separate the DC from AC path
and recombine as an architecture.
Good luck.
Regards, Dana.