At the risk of coming off insensitive, I would strongly recommend cremation. That would eliminate the risk of poisoning the soil, which is otherwise significant

That's fair. I would include the cremation into the fundraising exercise
I presume that Facebook is not yielding much in terms of donations?

Usually decent vets would give you some time to pay in a situation such as this
Will that guy not negotiate?

That's probably your best bet
20 minutes won't be enough time to fundraise whatever balance is left

Blood is much more difficult to provide, donations are easier, less friction
So I wouldn't discard Facebook. Post on the big groups with as much detail of the what, the when, the how and the why