After only 20K I couldn't stand the warped rotor, and PBR pads grinding when hot so I put new rotors and Volvo pads on. The job appears to be a success. However, inspection of the caliper indicated that there is a little gap about 3/4 inch long where the bottom edge of the dust boot pulled out. I couldn't get it to tuck back in. I tried pushing on it but couldn't get the lip back past the piston. I didn't want to push too hard and poke a hole or rip it. Is there a method to get it back in without pulling the entire caliper? I just left it. Should I worry about just leaving it?

IHMO, yes you should fix it. The boot lip should fit into a machined groove inside the caliper. If it's out of position it will allow dirt to get into the caliper cylinder and will interfere with piston return when brakes are released. The piston needs to come out of the cylinder in order to get the seal back in place. Guess you could do this with the caliper on, but it'd probably be easier with it off.