Firstly, you can access the inside by pulling the trunk liner out.
Secondly, you mention the paint is cracked. Even if the dent remover guy can punch it out, you still need address the cracks before moisture gets in and starts the rust process
If you're on your own on this repair, I'd try a large suction cup on the outside and try to "pop" the dent. I have an old computer floor tile double cup tool that is great for this sort of work, but a properly used plumber's plunger can be amazingly effective (wet the rim for a seal).
Even popping it out, chances are the metal is stretched and will remain wavy after you're finished. If commit to refinishing the panel, a shop can heat shrink the metal and straighten it out with a minimum of bondo.
Bummer, dude....
Jim