I think as Hiddy has said you need to draft an appropriate letter to BA.
Emphasise you appreciate that in the present financial climate you understand that they will have to cancel flights . Re-iterate that you will absorb the extra cost to yourself of the additional hotel night (even though you haven't budgeted for it.)
I would say to them that your ideal scenario would be travelling back on the same date as planned, but if there's no way they can facilitate you, the above is the only acceptable alternative.
I think I'd throw in a compounding reason why you can't come back a day earlier...use your imagination.
Was the BA person not suggesting cancelling the redemption and re-booking? Have you looked into this using the alternative itinerary?
Good luck anyway...hope everything else turns out OK^^