Security of S/MIME in case of CA compromise
No, since

compromising the communication $\:\implies\:$ compromising the PKI setup $\:\implies$

falsely claiming that a party generated a particular public key $\:\implies\:$ "if Alice … immediately."

.

Yes, we might assume that.

Note that, if the channel can carry voice, then faking a comparison can require voice impersonation.

This is the idea behind ZRTP (which admittedly is not for email).