Ed Greenwood hated the 4e FR and is enthusiastically on board with the Sundering, whatever that implies.
He’s actually not on record saying he “hated” the 4E Realms.
We know he was seriously putout when the changes were introduced to him, but he stuck with the setting by, amongst other things, writing the Waterdeep bible (used by the authors of the Ed Greenwood Presents: Waterdeep novels), doing work on Returned Abeir (i.e. Laerakond) and penning a long string of Eye on the Realms articles for Dungeon and later Dragon magazine (some 50 articles and counting).
FWIW, in the long history of the published Realms, TSR and WotC have both done things Greenwood has said he wouldn’t have done, were he still in charge.
I thought I heard the 5e Realms were going to support both the non-4e version and the 4e version so I assumed the Sundering would create two parallel Realms, one in which the SP happened and another where it didn't.
There is only one continuous timeline.
The Sundering is being introduced in a series of novels, the third of which is by Erin M. Evans (called “The Adversary”) and is available now, as I understand.
When those novels are all done, the setting will have advanced about ten years, to 1489 DR.
If it wasn't for R A Salvatore and his "novels" I suspect it would have been a rewind to 1375DR.
Not very likely.
WotC are the ones that approached Salvatore, Greenwood, et al., and asked them in so many words, “How do we fix the Realms?” Note these authors (not just Savlatore) have ongoing stories in the Realms and one of the Realms’ major themes is that it changes. It’s not a static place.
Stories are told in a linear fashion and there’s no evidence anyone wanted to jump back in time. The fix for the Realms involves moving the timeline forward.
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Other info:
The Sundering is supposed to be the very last Realms Shaking Event: something meant to put the world back in order.
The recent adventures all have rules downloads available that let DMs run their games using 3E, 4E and Next.
The intent of the Sundering is to get the old school Realms feel back. Greenwood stated in a recent podcast that changing the Realms should be done in a way that keeps it familiar to longtime fans (and so not in an abrupt manner, as was done with the Spellplague).
I’m pretty sure Akanul and Tymanther are going to be 86’d back to Abeir (which is unfortunate) and Anauroch will slowly return to swallow the Land Under Shadow (Netheril).