Firstly, congrats to your DD on an amazing set of offers. She has done brilliantly.
I totally understand her disappointment, however. (They're only 10 or 11, and they're human - of course they're going to have a favourite school however much we advise them not to).
Re the WL. The general assumption is that SPGS tend not to over-offer much (because so few people turn them down), but in past years, there have been a handful of girls who've got in on the WL. (There are already two parents on MN who are turning down SPGS places). But the main point of rejection will be after March 1st when state school offers come out (as there's some crossover between SPGS and Tiffins, and may parents will chose Tiffins and save themselves £300k!) Hopefully those parents will release before the deposit deadline, but sometimes it'll take time to trickle through, and it may not be before March 6th. (Remember if you turn down NLCS, someone else is hungrily waiting for that spot too, and they don't want to pay double deposit either!)
For now, I would call the Admissions office and ask where she is on the WL as a way of managing expectations (if she's number 20 then realistically there's probably not a huge chance, I imagine) and impress on them that it's DD's first choice.
Re double deposit, there's an argument that in the grand scheme of things - ie 7 years of fees at any of these schools - it's worth sucking up, but that's obviously for every family to decide based on their own circumstances.
Good luck and keep us posted!