@IRM I don't think it's possible to JG her CE after initially blocking it. And if it is, then its extremely difficult to do so.
It is indeed common knowledge about multi-hit CE's, but I don't think many people know about Viola's, especially the difference that with Viola's it's not just after the first hit that it can be GI'd, but after the first several hits. If the CE whiffs you need to wait a little while before running in, and not just wait for the first hit to whiff.
Knowing about this can be a good way of getting in on Viola, because if she whiffs her CE and you run in to impact it, she will get the Guard Impact stun regardless of what she's doing, even if she's holding guard.
It depends. For some like Pyrrha or Maxi the followups only come out when you block the initial hit. The blockstun would make it impossible to use GI however, so JG is a requirement. For others like Yoshi, Sieg or Viola, the followup hits come out always, so for them there is no need to JG. You could for example make them whiff the first hit and then GI, like Oof did in this video.