You're right that UEB does not include the "bidirectional arrow over" (which can denote a line) in the list of "common modifiers" (GTM 12).
So, the following would be appropriate transcriptions for your two print examples ("bidirectional arrow over BC" and "bidirectional arrow over CB"). I did "build" these using the directly above indicator (GTM 7).
The braille grouping indicators make the bidirectional arrow apply to both the B and C instead of just the C.
A grade 1 word indicator precedes the symbols-sequence, because otherwise it would mean "ghBCAR" and so on.
For our convenience, I've used a grade 1 word indicator for each of the examples above. In your full transcription, grade 1 mode would need to be applied in a way that works best in each situation.