1. I think u r right. in BJT , we should consider current gain, not just voltage gain which I addressed before.
2. if u want to route ur bandgap reference in a long path, u should use buffer after the bandgap core, not just put Vbg at everwhere in IC. that's not a pratical method.
3. i'm wrong in the statement, the PNPs are not in saturation, they are in the active region. afterall, BJT's operation is not the same as MOS. so since the BJT are in active region, their ro should be large, and so does the rejection will be good from collector to emitter.
5. I have done some simulations about the addition of Rb .
5-1 voltage noise:
if no Rb present, the voltage gain from Base to emitter is about -1dB, after add resistor range from 1 ~ 10k, the voltage gain is still about -1db.
5-2 current noise :
the current gain at emitter is about -120dB without Rb. and with Rb=1 ~ 1k, the gain is about -6db at Q1's emitter, and about 6db at Q2's emitter.
5-3, the voltage gain from collector to emitter is about -80db, which make sense, cause as state in 3, the PNP is really in active region.
5-4 apply voltage noise at the GND , and find response at emitters, the gain is about -1dB with Rb=1~10k ===> it is clearly dominated by 5-1.
5-5, if my simulation is correct, the addition of Rb has no improvement in noise reduction from GND.
6. by the way, in my simulations, the adding Rb can change the bandgap voltage.