Use either "shaded" mode instead of "Realistic" or turn on progressive refinement. The grainy transparency is a result of "Realistic" mode being essentially a progressive renderer, which refines the output over time/render passes. For editing and modelling it's best to work in "Shaded" mode, for lighting it's best to use…
To show the high res textures you need to set the viewport to "Realistic" and then go to Views > Show Materials In Viewport As > Realistic Materials with Maps. Have you tried this?
The sculpt is looking nice! Are you planning to go in a cartoony direction or a realistic one? The sculpt suggests realistic but this type of environment would be great in a TF2 or WoW style.
DAMN Rens!! \o/ Did you PAINT that photo-realistic beauty?? That's a lot of love that went in there! If you painted that hair, than that's CRAZY realistic! :O Awesome!!
My advice would be to make sure you have realistic expectations, and an efficient workflow-- before introducing UDIMs at all, let alone more UDIMs. UDIMs aren't a magic fix-all patch, you still need strong fundamentals or you'll end up working wastefully, and producing assets that are unnecessarily heavy. Just my two cents!