Understood where the 12V battery is. But if your jump starter is in the trunk, and you can't open the hatch, the only way to get to it is through the passenger compartment. Have you tried to do that? You may not even be able to get to that part of the trunk with the hatch down, particularly if you have your "coffin" in the lower area.
I don't own my FFE any more, and my son has the RS in Alabama, so can't check to see how hard it is to get into the trunk area from the passenger compartment. But I'm guessing it isn't easy.
GONE - 2015 FFE, Magnetic with leather interior, debadged & tinted
38,500 miles (since May 2015), average 246 Wh/mile
REPLACED WITH - 2018 Tesla Model3, LR RWD, Obsidian Black
20,000 miles (since Nov 2018), average 235 Wh/mile