A:AnswerThere is no sleep mode, turn the mouse off when you're not using it with the switch o. The bottom. The keyboard doesn't use much battery power when it's not being used
A:AnswerThe USB dongle allows you to run Bluetooth w/o additional computer hardware. It presumably downloads the required drivers. My PC doesn’t have built in Bluetooth, but I had a prior MS Bluetooth keyboard & mouse.
A:Answershape of keys (850 are indented, 900 are flat) and the space around the keys (850 is a more open design prone to crumb attaks. 2 AA batteries in the 900 mouse versus only 1 in the 850 mouse.