"Hah! Unless the key of B# is the 5th (V or V7) (or indeed any other related key essential for the chordal progression) of the Base Key of the Progression..."

Sorry FT, nice try but no cigar.

I'm talking about keys, not chords.



And I'm sorry to have to point out that even in the context of your error, B#, as a V chord, would be the V in the key of E# ... better described as F ...

So yours would be another entirely unrealistic scenario. :-)

... oh go on ... you can have a cigar anyway ...