I was also thinking it didn't make sense to have a bulkhead without anything behind it. Good that you found that out now than later.
With the vent there, it's impossible to have a corner tub there. I won't move the vent if the fireplace will ever be used. Oval tub is not bad, and with your new plan H6, you can have something decorative on the new bulkhead. With an oval tub, you probably will make a bulkhead anyway.
Some shower door hinges will stop at 90 degrees. Or you can have a door bumper on the wall/corner.
If you do make a wall next to the shower bench, I think you'll need lighting in the shower. If you have all glass panels you probably can go without it, since the light from the rest of the bathroom can fill in the shower space.
Since you need to move the vanity over to the shower side a little bit, or make the vanity a little shorter, you may want to consider building the shower wall a little longer so that you can bring the vanity right next to it. You need 17" overhang a comfortable bench, and 22" to overhang the vanity, that's 5" more. What you gain is a bigger vanity, which you need if you want to have towers on top, (I'd say a 72" vanity is on the short side to accommodate a tower), and more storage in the cabinets. What you lose is the wall space for towels, a bench (really? it doesn't make sense to me because no one is going to sit in that narrow space though), and the open feel in the shower when you have more glass. You cannot have the sink too close to the shower anyway, which will limit the placement of your towers. And you definitely need plenty of light in the shower to keep it from a dark corner. So, this option has many of its own limitations. Well, I'm only throwing ideas here.
mediaman, it seems that your shower bench does not take up an entire shower wall? Is it on purpose or you just didn't change the bench size? I'd say a corner bench is fine, a bench on a full side of wall is fine, but a narrow space between the bench and shower wall/glass is not good.