there seems to be enough experience to get to level 7 or 8 (8 was even shown in trailers)
This can be missleading ...
We dont really know where they were on level 8 in that trailer ... it may be part of Act II.
The only thing we know for sure is that they were on Grymforge at level 6 ... wich would suggest that completing whole Surface, Underdark and Grymforge content will get you on level 6 (or abowe, but still not to 7) ... and that sounds reasonable to me.
You have to remember that in DnD each next level cost aproximately twice as much XP as previous one ...
So technicaly, if you are moving around same location, geting from 5->6 should take twice as much time as 4->5 did.
Some XP gain tuning is to be expected ...
But i wouldnt presume any drastic changes.
And yes, max leve 12 seems most realistic, but nobody really know, except Larian ... and they are not talking.