I so sorry this has happen, Karen. Stay strong for her.
From my understanding, your vet if definitely WRONG to pts so soon. The longer she's alive, the more she'll heal.
Feeling pain in the back legs must indicate no severe spinal damage, otherwise her back legs would be numb and paralysed. I wonder whether she has broken her legs, or damaged her pelvis. Both of these CAN be treated successfully.
Unless she has serious internal damage, she should get through this.
The main thing is to give her time. Don't let the vet pts.
Unfortunately some vets are less willing to try hard for cats than they are for dogs. He may be that type. Insist that he gives her longer, at least a couple of weeks, maybe longer than that. This will provide more chance to assess her condition and for her to fight back, and fight she will, she knows she has kittens.
I don't like the sound of your vet, and if it was possible, I'd consult someone else. Moving her may not be wise, but this guy doesn't seem willing to put himself out for her. Any chance of another vet? Even if you have to take her by taxi and incur enormous costs, she's insured, so you know the vet fees will be paid.
It's hard at such an upsetting time, but FIGHT for her. Don't be bullied.
Sending you positive vibes and best wishes.