In respect to the allegations, respond the police and advise what has happened.
However your post isn't particularly clear as to where the money has gone and how much was actually returned. I can see how an accusation might surface as it's not clear where the money has gone.
Also, their money doesn't sound like a true gift. They gave it for a purpose, and that purpose is now gone, they can expect their gift returned. In fact in your post you advised family member made loans, are these the same members who are now making the accusations?
But from what I can understand from your post money wise:
The trip must be 3000, as the 10% they retain is only 300.
Assuming it was all paid, you got 2700 back.
On top of this, some family members chose to pay for stuff to help fundraise - may be baking supplies etc.
With the 2700 did you pay off everyone that lent the money for this?
As well as supplies, other members have also bought her equipment for the adventure. Was this returned to store and the money back to the owner? How much weren't you able to get back?
Essentially, did you spend more on the fundraising that you actually got back from fundraising and subsequently your refund?
And then on top of that you bought equipment, from loans, rather than making more through fundraising?
While Grandma was aware there was risk of financial loss, was she expecting you were taking that on, or did you make it clear to all involved you were asking them to take on risk as well?
While your daughter may have cut contact for unrelated reasons, that's no reason to ignore any debts. And if I read it correct, though you aren't related, there is no reason for you to ignore either.
I'd be a bit pissed too if I'd given money, then the recipient didn't complete their adventure, didn't pay me back, then go no contact, even if for unrelated reasons. Bit hard to ask a brick wall nicely either for your money back, hence I suppose, the police.
I get some money has been lost, as was the risk, but how much? You say you personally spent hundreds, did you get that back or have you lost that? If you've got it back and others have not, you should have made sure everyone else was settled up first, it's first and foremost your daughter, and so you must bear the most risk.
Donation is strange, if someone doesn't go on their fun run, I wouldn't expect to pay my fiver. But for the sake of the fiver, if I'd given it I wouldn't ask for it back. But bigger donations are more tricky, ultimately they'll have given it because your DD was going to do it. Not for the cause itself, iyswim. Now your DD isn't doing it, they don't want to give as much for the cause alone, as selfish as that sounds.