So every address generated by the wallet gets used exactly twice: once when the BTC are sent to the address and once when they are spent.
Even if the user received dozens of payments to the address? The wallet spends ALL those outputs at once? Doesn't that result in expensive transaction fees?
The fundamental assumption is that the address only gets one payment. If you screw with the fundamental assumption and hand out an address for multiple payments then I am not sure. I have never done that with my Trezor. Would be an interesting experiment I guess.