I’ve never been sure that more than 2 through platforms at Oxford Road has ever been of any benefit for this reason
Back when train lengths were shorter, everything could use half a platform and this worked well. You could have trains simultaneously arriving and departing from alternate platforms and barely anything used platform 5.
This is now no longer feasible because of the extension of trains to 5 or 6 coaches in many cases.
Eliminating reversals in platform 5 that foul all 4 other platforms and crew changes in such an illogical location is probably of more benefit, but Castlefield seriously does need to be 4 tracks throughout.
The crew changes aren't illogical and eliminating them would require a huge amount of route learning, which would take a long time to undertake. I agree that it would be desirable but the Treasury is unlikely to see it that way.
Castlefield being 2 track isn't really the biggest constraint, it is the flat junctions and insufficient platforms that kill it.
How does this make sense? Surely there would be more usable length on the centre platform without any taken up by buffers? Platform 1 urgently needs to be closed due to it being inaccessible which is simply not acceptable for a city centre station.
Because you need an overlap if there isn't a buffer. And that overlap would have to go onto either the Down line (thus preventing trains from arriving into Oxford Road from Piccadilly) or onto the Up line (thus preventing trains from departing Oxford Road for Piccadilly).
Otherwise a train couldn't arrive into the bay until the previous train had left the Up platform and vacated the overlap - in other words, the platform would be basically useless.