#1 I measured and marked it with a line 2cm all around the head of the stem (it is aluminium so it's soft metal) and followed the line while sawing. To get it even perfectly I grinded it afterwards first with a grindstone and then with a metal file, levelling it thoroughly on a flat surface. Painted the cut with Revell Aqua color acryl paint black matt which is the colour of my Vado. Sounds more work than it was.
#2 Removing the bar from the original stem wasn't a problem but once you have put the 8cm stem riser on top of it, it wasn#t possible to refit the bar and brakes over it. As said before you add the stem riser i.e. you put it in between. My cables were too short. YRMV.
#3 I bought it it Germany where I live but the bike24-shop sends to the US too:
https://www.bike24.de/p1373648.html
I've added marks to my attached pic:
the red marks the top part of the Ergotec stem I had cut by hand metal saw before the pic, and the yellow is a further cut I made after the pic was taken when I was shure I wouldn't need the further rise potential it represented (there are several spacer rings so you can adjust the Ergotec stem to the rise you need).