I am going to suggest a wiled guess.
See the attached pic. Get the boot on the piston as in the picture. Lube the bare piston between the boot and the heat shield with that special caliper lube they sell at places like Autozone.

The boot lip on the caliper body needs to be degreased with brake cleaner.

Degrease the inner boot lip the end that goes onto the caliper with brake cleaner.
Not shown in the pic put some lube on your finger and lube the inner caliper O-ring seal but not the end of the piston.

That should give you working space to push down on the caliper end boot lip to secure it to the caliper lip.

If you get the caliper end of the boot to stay in place let it sit a few hours to be sure it won't pop back off. After that you are going to have to do what is needed to get the heat shield end of the boot up into the groove.

I think if the bare piston area was well lubed and you slowly push in the piston the boot is going to slide at least partly to where it should go. then you can pull the Piston out a bit and see if you can get the boot all the way in the groove.

I know some people hate this Idea but this is where having the two sections of the Caliper taken apart is a huge help in giving you room to work in.

Push the Piston into the seal just barely; enough to get it centered.

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84 300D, 82 Volvo 244Gl Diesel