If its possible for them to be in first person then im confused as to why the conclusion is instantly drawn that third person is occasionally forced. It seems much more logical that he chose to switch to third person to show it off, being that the entire point of the videos is to show off the game.
The slow motion could also be related to the dragon shout that slows time, much like the infamous squating with a sword drawn.
Your views could be correct too (and if they are im pretty worried) but you should consider other factors like the ones i mentioned as well.
You can think of other explanations as much as you want, but I go by what other people (that have seen and heard Bethesda at demos and other hearings) has actually stated (confirmed) in reviews and magazines. I'm also going by what's most likely and what it looks like.
What is most likely? - It's incredibly common that finishing moves and such in games force the player into some cinematic view, usually... yeah third-person.
- Bethesda has shown interest in cinematic camera views (that force the player into third-person when you use it) like this before for Fallout 3 (V.A.T.S.)
- Is it also more likely that Bethesda (at two different times) went out to third-person quickly during the finishing moves, instead of them actually forcing you into third-person? I wouldn't say it is, especially consider the above two points as well.
If you still doubt (personally it's 100% clear to me), then it's simply to put a "what WOULD you like/dislike, IF it was this..." before your sentences.