Riding that high off level will not be good on the tires on the fifth's rear axle, they will be taking most of the weight.
The pin boxes can be modified (sometimes) by drilling more matching holes in the king pin sleeve and pushing it further in. If there is an interference at the top of the sleeve then a portion of it can be (plasma) cut off.
I don't think it's a solution in your situation since you are already close to the truck bed sidewalls, so you have to raise the fifth. If you are off level by 6" unfortunately it means it will have to go up by 6", it doesn't matter what your wheelbase is from the pin to the center of the axles. That's a lot. 4" is more typical in those situations, you might have to do a combination of the pinbox mod and axle mod. Good competent frame shop (don't go to a dealer, they'll just send it out and skin you alive) should be able to do it. Depending what the Domani frame is a section square or rectangular tubing could be welded in the area where the axles are and axles re-attached below that added tubing. Being square to the frame and to each other (axle) is very important here. Torqueflex suspension is indeed a good suspension in a dual axle setup, but it's also very finicky when it comes to installation, Dexter has a whole bunch of pages on how to "properly" install those, this might be a reason why the suspension company wants to switch to a conventional spring setup. I would not be too "disappointed" with a spring axles, these have been around for a long time and they work very well. I would guess that the shop wants to do what is known as an "axle flip", I had it done on my fifth to go from a very low setup with a pickup (19 inch tires),
to pulling with a semi (22 inch tires).
You can see in the second picture that the tires on the fifth are not as deep into the well (4 inches lower). What the "axle flip" does it mounts the springs " overslung" vs. "underslung" you might want to discuss this terminology with the suspension company. There are two issues with doing this, the tires will be lower in the well and I don't know how funky it might look on a Domani, maybe not funky at all. It certainly didn't make any difference (in the "look") on my rig (16 inch fifth tires). Second your rig will be higher and the steps might not work as well. On mine I solved the problem with an "accessory step",
which we add to the steps every time we set up. Pretty cheap from the RV parts supplier.