No help to you now as I have moved to Germany but I used to live in Manchester and would have gladly given you a roof for the full three days.
With regards to B&B's I can't really give you much advice as I never used any with having my home there but I would probably recommend Travel Lodge's or something similiar. As the MEN is on the corner of Trinity way and Victoria Street you'll probably want something fairly close to there. A quick search turned up
THIS and while I have never stayed there its location is pretty good to the city centre (5 mins) Salford Quays (2mins) and MEN (5mins).
Also for extra information...directly opposite the entrance to the MEN car park is Dutton Street. Providing you get there at the right time (early enough if there is a event on) then you can park for free, do not leave any valuables in the car though and especially not camera/laptop etc. The whole of Manchester is pretty bad for car crime and its simply not worth the risk. Finally on event day you will have lots of men along Trinity Way advertising Car parking for the Event, ignore them completely and go to either Dutton Street or use the MEN car park, the latter while not cheap is still cheaper than the rouges on Trinity Way and is secure during the event.
Hope that helps and if you need any more info just shout, I'll be glad to help were I can
