So mean what happens every time there is a loose ball?
Yes. This is pretty much what happens every time there is a loose ball, and players do ignore the whistle. If there were multiple whistles from at least 2 different officials, (and this pretty much also happens every time), then players at the edge of the pile should know not to continue.
I'm not in favour of continuing plays after the whistle or after the player/ball has touched out-of-bounds. This is a sport that creates a lot of 'what if...'.
As i've said multiple times i'm fine with the outcome of the player, which was officiated correctly. Also, (as I believe you pointed out), a backwards pass is NOT a fumble even though it is still a live ball ball. I knew that it was a live ball but I had forgotten that it wasn't - by rule - a fumble. If it had been classed as a fumble then - by rule - the 2-point try would have failed.
As to a wider point I was looking as to what the NFL could do with the rules if they wanted to prevent a repeat of this scoring play. Re-classifying a backwards pass as a fumble would actually create more problems than it would solve, (which is presumably why footballing authorities haven't done so). The only way to do would be to classify passes that are tipped by opponents and fall incomplete in advance of the point of the throw as incomplete forward passes.