But I don't know if a bus journey actually cancels an outerchange, or
if it would still count if it was within the time limit.

a possible related case:
what happens if someone on pre-pay hops on the 73/30/205/etc etc to
perform their outerchange between Euston Square and Euston ?
what happens if someone with a bus pass (who is using pre-pay for an
unaccustomed tube journey) does the same?