I can't imagine that they would refuse further donations, but regarding the time limit, my guess is that they would want as many submissions as possible to be made by the end of that time for budgeting purposes. Otherwise, they could potentially wait forever as donations continue to trickle in.
As for your personal circumstances, I wonder... I realise it may be hard to distinguish between genuine participants and people who just want to put their hand up for the sake of it, but given that RSI, et al, are working so closely with the gaming communities, perhaps there could be extensions granted in particular cases? Just a thought - I don't know how the financial side of things are managed.