El Cid

I had a Ford pick-up and searched on internet for good independent. Also asked several very reliable people. All recommended one place above all others.
Bottom line, I had to take it to dealer and pay $400 to fix what independent broke and then denied having done it.
When you deal with an independent, remember there is no one to appeal to or complain to. With an MB dealership, you have the service advisor, the service manager, the general manager, the owner and MBUSA.
Independents also lower costs by skipping some checks and they are not fully connected to the MBUSA/DAG computer information systems.
Oh and internet reviews are not very valid. Most are people who really dislike the place or really love it. Some with negative experiences may not post a rating for concerns with the place taking some kind of action.

squid23

OP, somebody here posted the exact conditions for heated seats to stay on just recently. It is a big pile, one issue being battery charge as perceived by the car.

I think that your car has been sitting for a while before you started driving it and various electric subsystems "change status" as it goes from just sitting rotting a bit to the hot/cold cycles that using a car brings with it. That is why it changed since your test drive. Normally it goes better but hey.

I would make double sure your battery is at the top of its game, and the alternator, too.

I am not that familiar with Mercedes' diagnostic subsystems, but I bet somebody could look specifically for codes indicating that the car is "careful" about battery charge.

El Cid

My car has sat for a week or more and heated seats came on and stayed on for normal length with a four year old battery. One thing about the seats is that MB engineers programmed them to only stay on for a certain time (15 mins.?). If you start on high, it cycles down to medium and then low and then off.
Asian cars, the heated seats stay on until you turn them off.