Just wondering if the 139 floats were flight tested at the flight speed limit or above with actual deployment of the floats? Per Part 29, the 92 was flight tested with deployment of the inadvertant deployment of the floats to speeds above 80 KIAS but that speed was referenced since it is BROC at SL. Benign handling quality changes with the deployments. Arming at high speeds only satisfys the fact that the floats can be armed anytime over water . I'm guessing that the 139 checklist still mentions arming the floats during routine and emergency checklists. AFDS only works when armed first so I agree that flying with armed floats is a great idea if over water but RFM limitations still apply.
The mindset of "it has never happened before" is asking for trouble. There have been many instances of inadvertent float deployments and there always will be. Riding a bucking bronco during inadvertant deployment at high speed is not my idea of fun. Don't think the engineers have completely eliminated the chances of system failures yet.
Since the 92 imersion switches are mounted in the sponsons, breakaway of the sponsons could have negated the auto deployment feature due to severed wires. Sea State 5 bags located on either side of the fwd fuselage and one on the tailboom, State 6 bags add bags on the sponsons. Lots of wire between switches and inflation bottles. I have seen first hand where auto systems have not worked due to the compromise of the structure causing wire harnesses to break.
Also the bags are not tollerent during hard water impacts when inflated prior to water contact. Don't know of any float bags that are.
Like I mentioned in my post earlier, theory and speculation will be answered during the crash investigation. Examination of the float system components, inflation bottles, mounting points, bags and the position of the arming switch will provide the answers. The CVR/FDRs and HUMS will also provide much needed info.
My thoughts and prayers are with the families.