Would the heat dissolve the HA under my eyes?
Treatments to the forehead with Ulthera will not affect the filler you have put into the lower lids.
In theory, doing a focused ultrasound treatment (HIFU ultherapy) may melt fillers in the skin. In your case, given that the fillers are in the undereye area and you plan to do HIFU/ultherapy of the forehead then it would probably be OK to proceed