But I can't help to think that from a marketing point of view it is better to not make a game set in a province that already exists in another game.
If TESO gives us all of Tamriel (judging from the announcement it will), do you really think TES VI will be set in one of the provinces that already exists in TESO? I personally hope the MMO and single player games will be truly separate, but the marketing team of BGS could very well have a different view. TES fans do not mind a province that has already been done, especially because it's a completely different time frame. But your average Joe when hearing about TES VI: Hammerfell (random example) might think "I already know that from TESO". It's bad from a marketing point of view.
I wouldn't be surprised if TES VI will not feature one of the provinces of Tamriel, but something different. Or that TES VI won't come before 2020 at which point TESO is pretty outdated.

both instances being developed for years, both instances suffering for said transfer (assumed)
Bethesda will keep working on their own game, maybe TES maybe not but I don't think we'll get an usually long time in between Skyrim and whatever comes next for TES. We know nothing about this MMO yet but it sounds like you are assuming it will be A bigger version of Skyrim with a lot of other people in it, which would be highly inaccurate. There's so much that could and will differ between the SP TES and this MMO. By saying no one but real fans would want to play a an already visited province during a SP experience is completely illogical. A game set in High Rock or Hammerfell would sell extremely well. Why? Because its a TES game and people are going to want to play it. Hell they could go with Morrowind again and it would sell like crazy. You also seem to forget that a humongous portion of TES fans will not be playing this MMO especially if it isn't on consoles.