In my opinion ui is not properly a diphthong (except in huius, cuius, huic, cui, and hui) Ecclesiastically, the ui in other words might well be slurred into "wih" or "wee" in speech (an example, I think, of synezesis). But in song the ui definitely represents two syllables. If one tries to sing the Regina Coeli with them slurred together one finds oneself short by a syllable with every meruisti.
Quia quem meruisti [meh-roo-IS-tee]
Meruisti portare
(Alleluia!)
Resurrexit sicut dixit,
Resurrexit sicut dixit,
Alleluia!
In meruisti the stress falls on the IS because the double consonant (st) makes that syllable heavy. This is not the case with meruit, so the accent would fall on the first syllable: MEH-roo-it.