I had AAA put 3 gallons in it to see.
It’s the right key.
With the Kia, turn the key to “on”, not start. You should hear a whine which is the electric fuel pump in the back of the car kicking in. I doubt the Kia has a mechanical fuel pump.
If there isn’t a whining sound, either the emergency shut off switch clicked off (happens to prevent fires in an accident) or the fuel pump needs to be replaced.
As far as the MBZ goes, I’ve owned four of them and never had a Key issue. Best guess the key is worn, since they used a regular type key back then rather than the electronic plug in type they use today. Either than, or the ignition switch needs to be replaced.
The only other option, have both cars towed to their respective Dealers and have them do the work.