Q1 Since the bodice is in Slip St pattern you'll need to work in multiples of 4+3. So you could use 63sts, as Sue suggests for the bodice which would mean decreasing one stitch from the 64sts you're going to use for the skirt. For the neck shaping, bind off more sts at the center neck and perhaps add in another decrease row at the neck. Finish with the shoulders a little more than 12sts wide, maybe 14 or 15sts. How much more depends on your gauge and the child's measurements.
Q2 You'll be able to shape the back using the shaping from the front as a guide. Leave the 5sts for the button and buttonhole bands. That should be fine.
Q3 Follow the armhole shaping as given in the pattern but perhaps extend the decreases for a row or two more.You'll have to pick up more sts for the sleeve depending on the measurement around the armhole. I'd err on the side of too wide a sleeve rather than too tight.
I think it would help if you write out the pattern with the changes you're going to make in the stitch numbers and the number of decrease rows, for example. Some of this, you're just going to have to try and see if it looks right, that is, proprtional for a child