What a fascinating topic, Kraig!
There are some sound changes that are much more common in one direction than
the reverse, and there are some sounds that are more resistant to change
than others. Others on the group will have much more detailed knowledge
than I - there's a branch of linguistics that looks at this sort of thing.
>Am I obligated to make the daughter language's cognate
> form be 'ría' or can I make it 'ru'
(a) You could argue "dialect" or "exception with explanation unknown at
present" (like Skt hrd "heart") or "substrate influence" or a number of such
things, but maybe those are somehow unsatisfying.
(b) You could invent a rule (like the RUKI rule in Slavic & I-I) so that /u/
was the outcome after /r/.
(c) Why do you want to make it different?
Peter