One suggestion that I have, maybe it will work maybe it won"t! Thats Life
There should be a facility for players to register prior to payments, something like a VIP list, these players would then have 72 hours to make their payments, if the payment fails I think this is plenty of time to get it sorted, this may stop the clickfest.
You could either have players register their email on a list and be invited, this could be done in 2 stages, for instance, first invitations go to players from the previous event, or alternatively players that register on a list on a first-come-first served basis, I also think that the facility to pre-register via BlueSquare would make the process easier. I know this opens a can of worms but maybe it would be easier than a hit or miss registration process at present.
By no means is any of this the perfect solution, my opinion is to have a 3 day event to allow more players and not to sell out. Just food for thought