SMPS voltage feedback using op-amps
Yes and no -- you've identified the biggest potential problem.  Typical "general purpose" op-amps are generally fast enough for today's "general purpose" switchers; I'm assuming that your loop will be particularly slow based on the voltage and the UC3843.  Basically the bandwidth should be at least ten times the anticipated loop bandwidth of the supply.
I'm too lazy to check for you, but in addition to vetting the bandwidth of the as-built amplifier stages, make sure that it doesn't hit the rails, or suffer from phase inversion or slow recovery when it's common mode range is exceeded.  You'll also want to check voltage and current offsets (and the effect of current bias on your unbalanced op-amp inputs)
Probably not.  In general a capacitor across R6 would be a Bad Thing -- it would slow down the loop, and potentially destabilize it.  If you get close to production and you have really high-frequency noise getting into the system, then a cap across R6 that's small enough not to compromise loop stability may be necessary, but I'd consider that a desperate measure, to fix a problem that's usually better dealt with by careful board layout.