i didn't want to quote your whole posting but i second everything you said and i have the same concerns.
i hope that hc mode footage is easily distinguishable from normal mode but i don't hang my hopes high there. maybe if we all have some experience with skating and filming in hc we can more easily spot the differences but there should still be something watermark like. at this point only a handful of people knows first hand how hc mode feels and looks like.
i thought about this whole situation yesterday evening when i couldn't sleep and i thought about something along the lines of maybe a different looking vignette or no vignetting at all in hc mode. that could still be avoided with masking and cropping and i could see people doing it just for the sake of growing their e-peens.
this whole problem brings me to a point where i ask myself if i am taking this game too serious. in the end, it shouldn't matter which mode a trick was filmed in, if it is looking good and fulfills its role in the video. but if someone is bragging with it .... and here we are back at the beginning.
long story short: let's just hope that there is some easy to spot visual difference.