Re: Isigar vs Hill Giant
Thinking about this one - it's a little tricky, actually, but that's fine, that's what the arena is for. Let's start with the basics of Balance: if you're on a problematic surface, you roll a Balance check against the relevant DC and find yourself in one of three categories: can move at half-speed (success), can't move (fail by 4 or less), or can't move and fall prone (fail by 5 or more).
Now, the Impeding Stones spell takes a stricter approach, without the middle category: check against the spell DC, and either you can move at half-speed (success) or you can't move and fall prone (fail).
So, what happens if you fall prone, and then use a move action to stand? In that case, you are trying to move on a precarious surface, and I think at that point the normal, more lenient rules of the Balance skill apply - you roll a Balance check against the spell's DC, and you get the benefit of three categories: move at half-speed, can't move, or can't move and fall prone.
What happens if you fall prone, and then use a move action to crawl 5' without standing? Well - you are still trying to move on a precarious surface, so I think you still need to make a Balance check (there's no exception made for prone creatures). But you get +4 bonus for having four limbs to use, like a quadruped. If you succeed, you can move the 5' as per normal crawling, and if you fail, you just don't move (it doesn't matter whether you are deemed to "fall prone" again since you are already prone).
How does climbing figure into this? Well, climbing is a form of movement, at 1/4 normal speed. So, you could make a Balance check to let you move at half-speed, and climb at 1/8 of your normal base speed, presumably down to a minimum increment of 5' per move action. Or, given that the height of the wall is within the giant's reach, you could decide to just swing down and let go... So, without getting up from prone, if you succeed on your Balance checks you get to move two 5' squares on top of the wall's surface, at which point you're at the lip of the wall. At that point there should either be a Climb check (with a penalty for the shaking surface) or a Balance check plus a normal, unpenalized Climb check, to allow you to drop down safely, otherwise you take some falling damage.
This is all speculative of course, especially the interaction with the Climb skill. What do you think? (Do you know of any firm guidance on this?)