Hi T,
Yeah you should snap over 1 frame. It shouldn’t really be used as a slider but it’s way nicer for the animator to see it moving when they change this, it helps you recognise that there are 2 ‘positions/orientations’ the bone is in at all times (if that makes sense).
I usually don’t bother with IK/FK snapping, which is partly why the rig doesn’t have that feature but I think you might need to reset the rotations to zero on the FK bones while it is in IK. (That is because I was trying to be efficient with my rigging and used the same chain for FK and IK). This uses less bones and drivers (faster). But the downside is that the FK can still shift the IK when it’s in IK mode. Which is only a problem if the FK bones are animated while it is in IK (if they are held – it should be fine).

Hopefully you can make sense of drawing of what the GE might look like.
Let me know if any of that made sense haha