BOAC,
of course an interesting if not the key question.
One thing I was wondering is this:
going from CLB to IDLE to REV is a series of state changes rather than true thrust commands.
Maybe, and this is wild speculation as I will admit, maybe they had briefed the final stage (CVR recorded "reverse only one") and they have mentally skipped the "IDLE" stage for whatever reason and went right into REV. Of course, they had REV only mentally linked with the #1 engine so they pulled that lever.
Very very very speculative, but the only way of tunnel vision I can think of at this point.
pj