We all saw Snoke's dead face.
In Ep.VI we
also all saw Palpatine thrown into a bottomless pit (which helpfully exploded — twice — a couple of seconds/minutes later), yet the authors of the (now non-canon) New Republic-era novels were quite happy to have clone(?)-Palpatine rise from the dead to take command of the Imperial remnants.
And in Ep.I, we saw Darth Maul get cut into 2 pieces
and fall into a bottomless pit, from which it could reasonably be assumed that he was dead (or at least, that heee... would beee...!). Yet — since he was apparently too cool a character to be killed off just
once — the
Clone Wars scriptwriters found a way to bring him back.
I'm certainly not suggesting that Snoke was/is
cooler than Maul, but assuming that Snoke really is dead, does also pretty much imply that he was less Force-powerful than Maul was (though even Maul himself didn't manage to assure his own self-promotion by killing his Master!).
I'm also not directly agreeing with Gori's hypothesis (no offence, guy!), but rather simply pointing out that such resurrectional retcons do (unfortunately) have a precedent in the (Nu)SW continuity.
And finally, regarding what we the viewers believe to be true (or not):
Let's also not forget that back in 1980, every viewer of Ep.V clearly
heard Vader tell Luke "I am your father", but along with Luke, many viewers (apparently)
refused to believe it. And despite George Lucre* spending the following couple of years assuring people that Vader really
was telling the truth, yes he was, they continued to disbelieve it to the extent that he
still felt that it was necessary to add another couple of lines (scenes?) to the script of Ep.6, to show Luke fact-checking Vader's claim, and Yoda confirming it.
So I guess we'll just have to wait for Ep.IX...
*Yes, I know I mis-spelled it.