If you had two options for seating position and theoretically they both provide equal 'downforce' potential then you go with the one the driver prefers. So the influence isn't zero.
The thought experiment you have to run with this however is that if RBR was winning with the Merc seating position would Hamilton be calling for the same thing now? I'd take a punt and say confidently no. He'd assume he was better than max and would be fine with that position. It's only because RBR are winning with a car that appears to be more orientated towards his preference he can boldly claim about seating positions.
We're not privy as to what goes on at Merc, but if they are at the point of looking at their drivers for design suggestions then they really are in big trouble.
Has Hamilton actually said that the seat position is why the car is slow or has he only said that the seat position doesn’t allow him to get his usual feeling in the car?
It sounds like you’ve interpreted the former, when in fact he’s only said the later.
The car is slow because the car is slow (aero philosophy, concept, chassis, whatever). Team, engineers, drivers have all acknowledged that, but lap times more than anything. Regarding the characteristics of the car, Hamilton have George specifically said that it doesn’t have enough downforce. Hamilton has also specifically said that the aero balance shifts front to rear at different stages of the corner, from entry, mid to exit, in a poor way. These are what he’s attributed to the car being slow - global downforce and poor, unpredictable behaviour from corner entry to exit.
The two drivers are roughly separated by a tenth or so over almost two seasons and Hamilton has separately said that he can’t feel the car very well and he’s attributed that to the cockpit position. He hasn’t lumped everything together with its inherent pace. He’s specifically mentioned the cockpit position in relation to when George is able to get a notably better laptime out of it that he can’t. Lewis has said “George has shown that the lap time is possible but I can’t get the feeling out of it.,”
So to answer your little thought experiment based on the above. If the RBR was winning everything with a forward seat position, and George was getting a decent lap time out of the Merc while Lewis was on occasions 3 tenths or more behind at some weekends and he couldn’t feel the car, I suspect he will confirm the same i.e that he can’t feel the car. I really don’t understand why the mental gymnastics to try and interpret what a driver is saying other than what he is saying. We don’t actually know for certain if the seat position is affecting his feeling in the car, and we don’t know for certain it isn’t. But why start making aspersions that the driver is making up his perspective of it? His perspective IMO is one he genuinely believes, whether it’s correct or not.