Re alternative timelines - it is obviously part of fun speculating on new season (and there are elements of at least potentially retroactively changing universes, such as coop advising laura on not taking thering in fwwm post his lodge abduction, if it is indeed a post- one). But there do not seem to be too many real signs of that as of now - and even if taking say lost highway where we see a changed reality due to (most viably) fugue-type case as an example, an underlying illness if any
seems to be different in this case. One could maybe argue that you have potentially either psychosis or schizophrenia-type illness in coops case which makes him have psychoses or other sightings of things that do not exist.
I.e. his case, as the case of a presumed narrator from whose view the series is mostly experienced, would most likely seem to be some kind of crippling, but not inversing, an identity crisis-type state - and in any case, there always remained also a simple possibility of him say being mushroom poisoned by windom earle granting him a lodge 'access'. And even if taking fwwm's first part as cooper's conscience's rewrites or additions unto itself presumably after the events st the end of the series - thus btw indirectly confirming at least a POV anchoring of what is seen in the series with somebody actually present in it - do there really appear to be any other direct examples of a main reality/timeline shown being skewed to either side.
Otoh, paradoxically or not so much, seeing a town as an alive, breathing buffer of collective unconscious has always worked flawlessly in hindsight, no matter how many additions of hidden siblings/relational links or for that matter any other unknown past details ..