https://my.buick.com/content/dam/gmownercenter/gmna/dynamic/manuals/1996/buick/regal/1996_buick_regal_owners.pdf
That's the owners manual for this vehicle - refer to it for any operational questions.
From what I could tell, I do not see any indication that the rear seats of this vehicle fold down.
I do not believe you can get inside the tank from inside the car, see a local locksmith near you for assistance.
While looking in the owner’s manual check out jump starting - should have remote jump start terminals under hood - connect jumper cables - unlock trunk using release button.
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