Homegirl1, what do you mean by saying no places will be offered for waiting list with 7th deadline? Is it that if you have a deadline on the 7th for W offer, Trinity would not consider you off their waiting list at all, or is it that T are not able to tell you before the 7th whether they could give you a place?
If it's the latter and W are not willing to give you an extension, if only by one day, would Trinity agree to let you know at the end of their working day on Monday where you stand with this?
If T agree to let you know on Monday 'end of play' and it turns out there is no chance for a place at T, you still have, say, half an hour to an hour to email a scanned copy of your acceptance form and a proof of deposit payment to W and you can then drive up to the school and deliver the form by hand at the reception. Even if you were to do it at 6pm, it still would have been delivered on the 7th... From what I recall, W did not specify a particular time on the 7th as a deadline, or did they?
One thing to bear in mind is that in the past there was a following statement in W's offer letter: "Whitgift is one of the group of 10 schools which have agreed not to consider boys for admission after their parents have firmly accepted a place at one of those schools." That group included T, DC and A among others. The rules may have changed, but if they haven't, it would mean that if you accepted W and then tried to take up a place from a waiting list at T, T would not have you anyway plus you would have lost your deposit and one term's fees if you withdrew from W, as pointed out above.