How does one gloss a case that has both locative and genitive meaning?

Given your examples, I'd recommend that you don't think of the case as GENITIVE at all but only a LOCATIVE with a metaphorical extension of property and possession.

You have a very good precedent with languages like Russian which expresses possession as location: SHE HAS A BOOK = A BOOK IS BY HER.

This sort of semantic extension is very common with cases. Plus, there's some (not implausible) argument that genitive should not be considered a case because it does not denote syntactic relationships at the clause level (but that's a discussion for another thread).