Leave the mortar out. Since the tub will be sitting on its 4 legs, it does not need mortar, and any mortar put under it will
likely slump and shrink away from the tub, thus accomplishing nothing. Fiberglass is a better sound insulator than mortar.
I do not understand your comments about tiling over the lip, since this tub doesn't have tiling lips, or the lack of ledgers,
since this tub requires a deck/ledger under all four sides. Are we talking about the same tub?