Hello Jeff,
You mention two names that are independent of each other:
(1) Malcavi appears in a few extracts from medieval Spanish sources, but Malcani is more commonly found there. Moreover, numerous Almalcani existed as well. In numerous documents these two letters, 'v' and 'n', are spelled very similarly. So the correct letter should be 'n' rather than 'v'. In principle, we cannot exclude that this name is related to Malca/Melca. Indeed, in the same area (covering Aragon & Navarre) we find a number of such pairs as: Ahoel & Ahuellani, Alcana & Alcanani, Almaça and Almacani (Almaçani?) etc.
The element -ani could be an Arabic adjectival suffix. (At least, in the Levantine Arabic, -âni was and is still used to construct names of inhabitants of various places [in such cases, the form without -âni is a toponym] or occupations [in such cases, the form without -âni is the object of work]. It is also present in a number of Syrian Jewish surnames.
(2) Malcavit has several variant spellings: Amalcavit, Abnalcavit, Almacavit, Amelcavit. To my mind, there is no reason to think that this name can be related to Malca/Melca.
Best,