I don't know what you guys are talking about, you absolutely can roll back to any version of the engine in Steam by right clicking, going to Properties, Betas, and then selecting the version of MV you want to roll back to. If you select a specific one it won't update. I used MV for a long time without updating it, so if you were forced to update it for it to run that's a bit strange. If that's truly the case then the beta selection serves very little actual purpose.
Rolling back however won't fix your nwjs issue, though. It'll just reinstall whatever nwjs version they were using for that version of MV. You'll have to do your edits/updates to it again.
I strongly encourage everybody working on a game to at the very least back up their files periodically, and you should also consider selecting the version of RPG Maker MV in Steam that you are currently developing your game for so that no updates like this mess you up in the future.