I am really sorry to hear all of that
. I would like to help or contribute to the trouble shooting so here goes (sorry if this is of absolutely no help...)
-I had a very strong and audible thumping/shudder in my old civic when a mechanic forgot to tighten a caliper bolt and it was working its way off. It happened under different circumstances than yours anyway though.
- Under hard braking in both of my old Civics there was a shudder. Or prolonged braking down hill (even if not fast...but just long duration...It was most likely riding the brakes)
- In my Fit the brakes do a quick shudder/thumping that is felt and heard if I hit a bump (mostly speed bumps) while on the brake.
Is it possible that you might be hitting a bump while going down the hill while on the brakes?....
oh speaking of which, I really need to tell my dealer about that one...whooops!
-Was that crack sound possibly a "snap" of the calipers adjusting?
I have heard that on all of my previous hondas including this fit (but not as much on the fit, but
I also don't use nearly as heavily the brakes on the Fit compared to my older cars)
-Could it be possibly a worn or broken suspension component?
Which when going down hill with the brakes engaged meets a certain resonance and the thumping/shudder occurs.
Could that "crack" sound have come from the transmission shifting back to 1st?
(crudely of course)
-what about a blown/faulty/worn strut?
-should I shut up while I am behind?
I doubt that any of this ^^^ helps but...ya never know, much stranger things have happened...
good luck on resolving this