I gave you the idea to try to downgrade so I would kinda feel responsible if it doesn't work.
(I really hope the below doesn't come across as me being 'Mr Bleeding Obvious'.)
The decision is obviously entirely up to you and you seem to have done your best to explore all avenues toward a solution.
Unless another mod steps in with some further words of wisdom, I guess it's time for you to think about consequences and how you're going to cope with whatever may eventuate from your actions.
Don't mean to be heavy, but you may be looking at a permanent problem - if that is not what you have already.
Seems to me from my fairly lay perpsective that you have one of two problems.
1. You have a problem with your nand because you missed a step attempting to upgrade to CFW 3.55 (as you described in your first post). Downgrading down to 3.15 might blow this problem out and you can start again.
2. You have a (so far), undiagnosed hardware issue (e.g. your PS3 may be on its way to a YLOD), which was only exacerbated by the fw issue (if that's at all possible). Downgrading from this position will only get you stuck in a service mode loop.
Again, a mod may overrule this entirely; I'm only trying to help/inform with what I know.
Hope this helps rather than hinders.
"We all agree your theory is crazy, but is it crazy enough?" - Neils Bohr