Having used the Ultra Lite with my 30# wing originally, I found that because of the way it is made, it does not have a specific place on my back where it feels "right". So I didn't find it problematic to shift it lower - just like with a plastic back pak or a simple harness. This is unlike the Freedom Plate, which seems to have a spot where it fits best and my SS Hammerhead which also seems to have a sweet spot. The 18# wing should just reduce weight for travel and keep things even more streamlined.
I am also "foot heavy" so I don't want to have my wing high on my back, hitting me in the neck. One thing I am doing is putting XS Pockets on the cambands, so I can keep the weight up high and not all on a belt.
If it does not work this way, I can see how you have yours set-up and will change mine over by lowering the wing and using the webbing on the plate and not the slots.
I also keep all my bpw's set-up so that they feel the same as far as trim. That is the great thing about using a bpw - by having them set-up properly the only difference is changing the plate material/STA when changing wetsuit thickness; I normally never have to change the weight on my waist - so maintaining trim is easy.
PS - after looking at your pictures - as long as you position the tank (like you do - and so do I) so that the crown is tight against the inner radius of the wing, the PRAM fits fine with no interference - and again, by lowering the plate on your back, you can position it wherever you want in relation to your neck.
Also, if you want the speed harness to work as intended - do not run it through both slots on the lower keeper. If you let it slide and work like an old plastic plate, it is much easier to get in and out of and get it as snug as you want.