If you are more comfy with that you can do so, the address you need to use in emonhub will be
/dev/ttyACM0 (assuming this is the first/only arduino connected) instead of the usual
/dev/ttyAMA0. The arduino will then get it’s supply via the usb to, this can be “iffy” on the Arduino, but the Pi’s have particularly stable 5v rails so less of an issue.
Yes, but as Robert has pointed out, it’s only the 5v tx line from the Uno to the 3.3v rx line of the Pi that needs reducing, so a voltage divider (2 resisters) is adequate.
That depends on your sketch in the Uno. Yes it’s pretty easy if your Uno is outputting either space separated variables or key:value pairs at each update. emonhub can take either, you just need make the sketch match for the easiest solution.