Bluetooth adapter might render switch unusable?

I'm pretty certain that the Bluetooth adapters that you plug into the USB-C plug at the bottom do not modify the software of the Switch so it should not be a problem.

This thing is really about opening your switch and doing things like modifying the internal components, or modify the software that is running on the switch. The bluetooth adapters, as far as I know, do neither. They use the fact that you can output sound via the USB-C plug and then convert that sound signal to bluetooth for your bluetooth headphones.

So yeah as long as the thing you buy does not request you physically open the Switch, I do not think that the blueetooth dongles cause a problem for the Switch TOS.