The weight of a baby fluctuates wildly from day to day. You'll need to take a couple of weights over a 24 hour period to get a baseline. +1 on the digital scale. I prefer the ones that measure in grams.
Yes. measure on the bottom of the shell. Front to back.
Two inches since the end of October is a lot of growing.
My Red Foot grow about 2.5 inches a year.