It seems like you have a defective seat - having problems with the seat in 1 week / 500 miles. Suggest you get them fixed under warranty. I have covered more than 10K miles on my M3 in the last 5 months, and I have not experienced any of the problems with the seats that you have mentioned.
Last month I completed a 2600 mile round trip in 8 days, including a trip up Brasstown Bald Mountain in Georgia. Drove in excess of 600 miles on a couple of days and did not experience any of the issues you are having, no fatigue, no backache. I am 5'11" and 180 lbs. The only issue I seemed to have with the seat, is that when I tried to take a quick nap during charging breaks, I did not find it too comfortable for a nap. But then I have the car to drive not to nap, so not making a big issue out of it. My TL was really comfortable for a nap.
As a matter of fact, my wife who has a bad back, and always had a problem after long trips in my previous 2015 Acura TLX and 2011 Acura TL had zero back ache even after a 2600 mile trip. In fact, she has already started planing another road trip for Spring, whereas earlier she preferred trips by Air instead. Looks like Tesla will turn her into a road warrior as well.
After reading your post, I specifically checked all over the seat, and could not feel any metal anywhere other than the bottom of the seat near the floor. Speeding through the twisting roads on the Brasstown Bald Mountain, did not feel any roll. I was too busy enjoying the exhilarating drive, and the super handling. No creases or blemishes anywhere on the seats after 5 months and 10K miles.
Yes, I agree that the seat could be slightly longer to provide support for the thigh, but it is not a deal breaker for me. My Acura TLX had the same problem, compared to the TL which had a longer seat. Also, the seat material is definitely not top class, it is not real leather, but then the M3 is not supposed to be a high end luxury vehicle, even though the price is.
I have been using leather seats since my 2005 Honda Accord days, yes I miss the feel of real leather, but the super handling and the features of the M3, more than make up for the lack of real leather here. I could spend another 1500 and buy Recaro seats, but I am staying with the original ones.