Unfortunately, he does not seem to elaborate thereon. Vācaspati elaborates extensively and discusses absent pots on the floor, the sheer floor and all we know after Kumārila. Why does Uddyotakara not elaborate thereon?

Probably because someone else (who?) in the tradition had mentioned this possibility and so readers would have understood what he meant by the mention of the sixth kind of contact.

Moreover, is Uddyotakara’s viśeṣaṇaviśeṣyabhāva the same as what will be later known as saṃyuktaviśeṣaṇatā ‘the fact of being an attribute of something being in contact [with the sense faculties]’?