Thanks but unfortunately the National Express was sold out. The invitation only came last week and we acted on it immediately - I think the football match coinciding is the problem otherwise we would stay at the Youth Hostel both nights

As I said - go elsewhere and stay the night.
The 5:50am coach - where does it stop? Could you get an earlier bus to where it stops en-route (eg the 19:40 which stops at Birmingham or Bristol).
It isn't rocket science - if you can't find somewhere to stay where you are, then go somewhere else where you can.
Knowing the interview time would help. National Express shows decent availability on 25th November for still reasonably priced tickets down to Birmingham where you could overnight. As an example, the 21:30 departing Manchester coach station (so just before a hoard of football fans head your way) arrives in Birmingham at 00:20. Depending on times, you could maybe make the 19:30 departure or even the 18:00. The first two are £11 total (not per person), the last is £8. Those fares aren't gonna last long.
That dumps you in Birmingham at Digbeth coach station. The Birmingham bull ring travelodge wants £66 for the night for the two of you; the next day prices are fairly sensible if you buy NE tickets from Birmingham to Bristol and then again from Bristol to wherever it is in Cornwall (I guessed at Newquay).
I'm coming late to this thread, but I used to live in Manchester and at that time of night I wouldn't stay out anywhere in the city centre. I wonder if the OP considered Airbnb.

Just a thought. You could catch the Metrolink light railway, say to Bury, and look for AirB&B or guest house/hotel accom. there, then simply journey back the same way to Manchester centre the day after. Bury would be too far out for MUFC fans- and your travelling the opposite direction.