Sounds like the stump might be the culprit in your case.
What will you do differently when you restart this? The rubber mat may or may not be a factor. Perhaps a thin layer of sand under the gravel might help dissipate the weight of the rocks? Just a suggestion.
If I have to restart completely, I think I'm going to go with egg crate just to be safe, leave out the osmocote, and probably buy Ecco-Complete instead of blasting sand, since that's what I did on a prior tank which worked fine.
As to the driftwood -- I don't know. Certainly fasten it down differently, but that's cosmetic. Maybe the problem was caused by it, I don't know. I like the piece. A Chlorine Soak probably won't solve it -- if it's decaying wood it may actually make it worse (if delay the onset). There's no bark, it's mostly hard to a fingernail test. Some of it is flaking off in thin areas, but not in a mushy way. In short it looks appropriate.
About the only thing I know for sure didn't cause it is the rocks. So if I have to literally empty the tank of everything, I probably won't put anything back but the rocks, and find driftwood elsewhere, rather than risk a repeat.
it's refilling now from about a 50% WC (I have storage for only about 30% but will just make more to fill the balance). I'll see how that goes for a few days.
If it's the matt + osmocote giving off ammonia at that rate I may be doomed. If it's decay products from the initial bloom, it will come down now and stay down until the cycle kicks in. I should know in a few days.
I still think the mat itself is OK -- it's been two weeks and just a trace of ammonia in a glass with very little water. But ... since it's useful to let others learn from our mistakes... I would still say "in an abundance of caution don't do that". Which of course is what everyone said to begin with.