I don't believe that's true. Once the DS has booted into your flashcart menu, you can remove the pass-device and it will continue to load DS games. So it can't be accessing the pass-device all the time. In fact, I've heard people remove the pass-device after booting in order to preserve their battery life.No. When you have a passcard/superkey/passkey2/ect.., the DS is constantly reading the NDS port because the pass device constantly has to redirect all data from the GBA port.
I don't know about everyone else, but I've used Flashme Stealth on my dsl and I have noticed a markedly lower battery life. With my usage, pre-flashme I usually have to recharge it once a week. After using flashme stealth, I need to charge it every 3 days. Of course, I have Brain Age in the ds slot for sleep mode, but I leave my sclite in my gba slot when I put it to sleep