Services could probably be extended to Bournemouth if absolutely necessary, or drivers could shuttle to work.
My proposal would keep roughly the same seat count between Reading-Birmingham, Bristol-Birmingham and Birmingham-Sheffield as per today, but concentrated onto one service per hour rather than two. However, the reduced frequency would reduce passenger numbers slightly, as more travel via other operators or alternate means. Overall I'd be confident that a longer, hourly train would be less overcrowded than half-hourly, short trains.
Additional Class 350 services from Birmingham-Manchester wouled increase capacity along that corridor and maybe an extra WMR Birmingham-Reading considered in the medium-term.
Obviously this isn't an ideal solution, the railway shouldn't be trying to limit passenger numbers like this. But the current situation isn't acceptable either IMO and getting a significant amount of extra rolling stock will take time, so something has to give.