they are either idiots or inexperienced if they can’t figure out how to get that off. You simply hammer a socket onto it and get it off. Also, check your glove box and center console, I have seen locking Lugnut keys in those places as well.
not only that, but an empty tank full of fumes will be far more combustible and dangerous than one full of gasoline. Neither one should be in your living space.
it sounds like this is their third guess, I would strongly recommend another dealer altogether. And don’t tell the next dealer that you just want them to change the fuel pump, you want them to diagnose the issue. That way they are on the hook for the fix. And push for warranty.
Car stereo guy here: I think it’s reasonable for manufacturers to charge for an app, since anything that allows tracking and cell phone control of your vehicle outside of Bluetooth is always subscription based. If it was a feature inside the car or off a regular remote control transmitter then that would be completely different. If the app is so poorly designed that it won’t work if you have half a tank of fuel or less, or if it seems prohibitively expensive like more than maybe $100 a year )and includes tracking), then check out car stereo shops in your area. There’s a company called Mid city engineering, there’s Compustar, and there is DEI/Viper, one or all may have a remote start product for your Mercedes. One time cost. If you want something that is app based they will have options then you will be back to a subscription, but it might be better value.