It really doesn’t matter how fast you’re driving when a timing chain or timing belt breaks. If the engine is an interference design, then this failure has possibly turned the rest of the engine to junk. That’s because the valves and pistons have tried to occupy the same space at the same time, and widespread internal damage is the usually outcome.

From what I can gather, the V6 engine in your car is indeed an interference design, so the damage is possibly terminal. You could remove the cylinder head(s) to check, but if the engine has locked up, there’s a fair chance you’ll need to either rebuild the engine or replace it with a different one. You may also find that the cost of these repairs will be greater than the market value of the vehicle itself. At that point, you need to decide whether to repair it or start again with a new vehicle.

A modern automatic transmission is a very complex piece of equipment with a whole bunch of parts that have to be talking to each other for it to work properly. Your best plan of attack is to have the vehicle inspected by a transmission specialist who is familiar with this transmission and should be able to diagnose the fault.

But it sounds like there’s something broken or loose inside that is not allowing the engine’s power to be transferred to the car’s wheels. This could be anything form an input or output shaft, a clutch assembly, torque converter or even the gearsets themselves. Perhaps the fault is in the valve body (the part that actually selects each gear) but until it’s torn down and inspected, you won’t know what’s going on.

The other major cause of a car not moving when it’s in gear is that there’s a broken driveshaft or CV joint. The symptoms can be very similar, so check the whole driveline before committing to repairs.

The Santa Fe may well have a fuel filter below the rear seat, but it’s almost certainly located on the outside of the car (as in, under the floor) rather than inside under the seat cushion. I’d be very surprised if any of the car’s fuel system or lines ran inside the passenger compartment.

Which means it’s still possible that some fuel was spilled under the car during the filter change or that there’s even a fuel leak as a result of the change. Either could lead to a smell inside the car, but it shouldn’t persist. If it does, there’s a problem. Take it back to the dealership and ask for the problem to be fixed. If the dealership somehow spilled petrol on the seat trim or allowed fuel to enter the cabin in some other way, they are responsible for fixing it. Driving around in a car with petrol fumes inside is an obvious health and safety issue.