I am sorry you lost your appointment. That stinks.
I wonder if your litter box is too small. I just switched from a jumbo corner one to a cat box and my guy likes it much better. Also, my bun wouldn’t use the litterbox with the grate on top consistently. I had to take it off. Do you put hay in the box? Maybe more of a screen with hay on top would be better than a grate? I am not sure the type of litter would change anything. Have you tried other litters? All I know is that when I tried a corner box with a grate my bun’s perfect litter habits stopped and he started peeing on the floor instead. He would still poo in the box but didn’t want to pee in it. Maybe other members would be able to help you with the digging/eating litter since mine doesn’t do that. As far as bonding, definitely get your girl spayed first before you introduce them. I would think that if you give her a few weeks (up to a month in some buns) to let all of her hormones regulate she should be fine to start bonding with your boy. I would hold off on introducing them until they are ready to start the bonding process because hormones can cause some negative reactions, even if your boy is fixed. I would hate them to hold a grudge before the bonding process even starts.