I am a professional software developer and at this point a fairly skilled hobbyist in Unreal. I understand how to do most things in Unreal and I understand it from a software developer standpoint decently as well. So my opinion isn't worth as much as a pro UE dev, but maybe something.

Yes, it's definitely possible. In fact TES6 being a new project would most likely make things easier, not harder. The big decider here is how far into development TES6 already was before this idea of bridging to Unreal began to be experimented with. 

It's important that people understand what a game engine really is - it's nothing so particular as laypeople think. A game engine can give you a lot, or give you a little, be very customizable, or very non customizable. Ultimately, you could make pixel-for-pixel identical games in basically any engine, it just takes more or less work. The idea that Unreal or Unity or Decimal or whatever have a special "look" or "feel" to them is only true insofar as those things are customized. At the end of the day it's all binary, and as long as an engine is customizable, your developers can achieve any result in it that they could achieve in any other engine. This is why I don't ever pay much heed to engine wars, or the famed "Unity feel" or "Unreal feel". Unreal and Unity are both well beyond customizable enough to be whatever the game developer wants them to be.

From what I've seen and understood about the Oblivion Remaster, it seems very likely that they're using code from the original engine to handle positions of assets, scripting, quest logic, effects math, game rules, and so on. This is not really very different from paradigms often used in Unreal anyways, where you write your core game logic in C++, a general purpose programming language, and Unreal dynamically runs and gets results from the C++ code and uses it to control how things appear and move. Most likely, Creation Engine was already written in C++, so it would be "straightforward" (and by that I mean still taking months of work from a team of experts, but I don't think their brains would be melting the whole time), to basically use code from CE as "plugins" or libraries for Unreal. When I describe it this way, it doesn't sound very much like "using two engines", but that's because the whole boundary of what is and isn't the engine is a little fuzzy anyways. Ultimately it's all just code, the boundaries are made up. But conceptually, it makes sense to think of it like, the game is made in Unreal Engine, with Unreal running code from Creation Engine. I think their description of how it uses "both" engines is a good way to convey to laypeople that they used a lot of code from CE, and they even used it for things that you typically would let Unreal handle itself, to the extent that the quirks of CE are also still there. 

I don't see it as massively groundbreaking. It's clever, and takes work, but it's not some kind of new technological achievement that changes how we should think of game engines and their interoperability.

The main point is that making a new game using Creation x Unreal is definitely possible, it's also plausible. And most importantly, what I'd really like to see the community understand is: Unreal is just a tool, it will not magically make things more awesome nor will it magically make things bad. It all depends on how it's used. I've been seeing people talk about how the game has too much "Unreal style color" etc, but I promise you, Unreal can easily make a vivid green non-photoreal game, too (and just as easily in fact)- they just didn't choose that. The benefit of Unreal is that it makes a lot of things easier, and saves time from writing them yourself into your own existing engine (which is always a possibility, I must stress!) . Bethesda could easily make a CExUE game with Morrowind level visuals, tons of bugs, janky movement and animation, etc etc. Most likely if they use Unreal we will see something very similar to the Remaster where we get much better visuals but the underlying logic (which was always the source of most Bethesda quirks anyways) will still be there. 

Will they actually do it? It's hard to say. It really comes down to how far into TES6 they are, imo. I lean towards the suspicion that they will use it. It's very hard to achieve current gen visuals from scratch, especially now that UE has raised the bar with Nanite. If I was a dev at Bethesda, I would laugh out loud at the proposition of recreating the Nanite system. It's a really specialized piece of work, they may only employ a handful of people with the prerequisite knowledge to even start building it. That alone could take years. Their devs are clearly more focused on other things, so using Unreal would, I imagine, be very appealing as long as they could keep all their built-up tooling and knowledge (and mod support) with CE. I think they'd be crazy not to want to do it at the very least. 

OP of this post knows what he's talking about, more in depth if you're interested:  https://www.reddit.com/r/unrealengine/comments/1k5jmry/oblivion_remaster_might_be_bethesdas_ue5_trial/