>The Latin thing is strange since cuyus is in fact
> inflected in Plautus.
Gender-marked forms of cuius (or quoius) as a possessive adjective are not
that rare. It is its use as a relative or an interrogative which is
unusual, and commonest in Plautus and Terence, but it can occur even in
Classical Latin (eg Vergil: cuium pecus?).
There are other possibilities beside "vacillating norm"
(a) The confusion of two forms: interrogative/relative cuius (all genders =
whose? or whose) and possessive adjective cuius -a -um (= and his/her/its)
(b) Uneducated speech reflected in writing: Plautus and Terence are both
comic writers, and Vergil's example is in the mouth of a country bumpkin.
Either way, it is more likely to be a development within Latin, than
something inherited.
Peter