“It looks like for ariya jhana, the higher the jhana, the more things cease.”
– Actually, that holds true for anariya jhana as well.
– However, it is not possible to enter Nirodha Samapatti, i.e., the last step is not possible with anariya jhana. That is because defilements are only SUPPRESSED in anariya jhana, whereas an Arahant has eliminated all defilements.
“We’ve also learned that Nirodha Samapatti can last a maximum of 7 days. From this I speculate that the physical body on its own can stay alive for 7 days without the Jīvitindriya cetasika?”
– That is correct. The physical body is kept alive by the jivindriya rupa, not the jivitindriya cetasika.