May be you can explain them that it's just delayed for one or two weeks because she is in heat. I don't think they will force you to do it now if they is more risk for the cat. That would be stupid from shelter people. I think they just want to make sure the adopted cats will really be spayed. But they could not know she would be in heat right after the adoption.

Scary vets are in your area, it seems they just want to do it for money!! The few vets I know just refuse to do it when the kitties are in heat.

I do not know your kitty except from the pictures, but I would be sad that something happens to her...