Are you looking at the smaller or larger Oliver? I assumed the smaller since you are comparing it to a Casita. One of the original designers, John Oliver, towed his Elite with a Nissan Frontier, so you should be fine. I chanced to meet him a number of years ago getting groceries during a solo hunting trip. Sweet rig! Very humble guy.
You might add
Bigfoot to your list. Their smallest 17.5’ model is a bit roomier that the Casita or smaller Oliver. It’s set up for four season use (like the Oliver but not Casita or Airstream). Very high build quality in a more traditional style. Downside is it’s a bit harder tow due to the added width and boxy shape, but still comfortably within your capabilities. I think you’d be okay on the interior height. Leaf-spring
axle set-up is easily modified for additional off-road clearance if desired.
Escape also makes a nice trailer in that size range. Its lightweight frame, like the Casita, is good for
light off-highway use only. Price-wise it slots between Casita and Oliver/Bigfoot/Airstream.
I’m with Bill on Airstream, and I really don’t see them as a good off road choice. If I really wanted one, I’d look for an older one that’s been redone.
Oliver is arguably the best on your list for off-road use due to the narrow width, strong aluminum frame, and good stock clearance. The all-fiberglass interior is a love-it-or-hate-it deal. Some find it clean and modern, others cold and sterile. Not sure about the height, but I don’t trust published specs. As you say, the size and placement of projections can make the difference.
Best wishes in your search!