yes could be fluid film or wool etc. i would get under there and scratch with your fingernail to make sure it's not rubberized coating. fluid film will come off like a greasy layer if relatively new or greasy/powdery cake if it's been on there for a while.
rubberized undercoating would be a red...
i haven't had my seats on in a few years but i dont recall ever having the headrest on them when i stowed them. looks to me like perhaps your headrest is causing the seat to be jammed with the back of the middle row.
just a guess
damn that looks like it's gonna blow any second.
given the risk of being stranded far away and possible overheated engine in the dead of summer i personally wouldnt chance it. if that happens you're talking potential head gasket and possible warped block etc etc.
that's a pretty long crack...
total guess but perhaps it's some sort of over-applied adhesive in the door gasket or liner that's melting with the heat? is it greasy (like the silicon grease on the sunroof tracks) or like water stain?
i don't recall ever seeing anyone post a similar issue but i've only been reading mud for...
i'm sorry to hear. i suggest putting a file together of your gas receipt or credit card statement, dash cam (anything else you have) showing the location, date and time of your incident, photos of the pump and nozzle etc to preserve flexibility should you decide to go back and see if the gas...
i'm not necessarily jumping to conclusions yet, you have certain things to verify first but if the gas station had the wrong dispenser nozzle on the diesel pump, here are a few thoughts to consider:
(also not sure if you're stateside and if your country has different rules etc)