They also said I should only use the chairs manufacturer parts
Why should you be made to do that?
I fitted a set of yamaha rims to my suzuki. And many aftermarket tuning parts like a rajay turbo...
I fitted a V8 from one manufacturer, into a different manufacturers car.
I fitted a different SSD hard disk, IN MY Dell laptop.
Rollex fitted my van with their own computer, and a floor and suspension system designed by them. As well as trim, loom, etc.
Nobody stops you, and in fact will go out of their way to help if they get to supply a part or extra.
Suzuki started fitting forged pistons in their 16V 4 cylindered GS1100 bikes with a billet crank, as stock to help the aftermarket tuning brigade since it helped sell more of THEIR bikes.
When I fitted nitrous to my bikes, the local suzuki dealer fitted me an AFTERMARKET billet clutch basket to cope with the extra power. And a different ignition module with a higher RPM limit at my request. They didnt give me a lecture about doubling the horsepower.
So what makes powerchairs so damned different?