Not all cell phones, BT headsets, etc. use USB connections, so they'd still need a standard power connector (and the 2mm one is better than the old one for headsets and the like; why not make it universal?).
And it makes very little sense to support charging only through the USB port, and make a USB connection (to charge) render the SD cards unavailable,
and then also allow the USB port to be used in host mode. This would actually get very annoying in practice.
It doesn't mean that; the two are unrelated. As for trying it, if I were you, I wouldn't. You don't seem to have a particularly extensive understanding of the tablets. There are some 770 schematics publicly available; it might be wise to look at them, even though the N800 is not the same, they'll give you a better idea if the mod you're considering is workable.