Once you start heating and trying to force the seats to soften yourself and cause any accidental damage, you’ll have then given them a reason not to take it back.
I’d let bmw and Motability sort it. It sounds like the basic seat has a fixed lumbar support right were you don’t want it.
mine on the Vw is electric and I switch it right down until I feel nothing at all.
If any of my family drive with the seat like this they think it feels strange. But they have no spine problems.