I am pretty sure Michael starts to treat scratches on Datu's face, but then says Datu is still bleeding out from somewhere else and finds the bite.

Ink does not seem to be able to upgrade existing biters to inklings. I think Datu was targeted to be a little one and was scratched. With the death of the inkling, Randy infects Datu with the standard biter infection.

Randy might just be a bumbling biter 1.0 and screwing everything up all the time. Alternatively, there is enough of him left inside trying to undermine Ink without making it too obvious.