Mominpink, you have the right idea. You do the BO on the WS like you said and at the end of the row dec 1 for the neck. And then on all the even rows (RS) you work a K2tog at the end where you did the 10 BO sts (until you have done the underarm deceases 9 more times). At the same time on every 4th row do the decrease for the neck side.
You are right on the first one, it would be on row 5, and it will always fall on a WS row. I'm not sure you counted right on the next decrease that you have for row 8. It would be on row 9. Row 8 will be a knit row and you want it on the next row. (you had two purl rows one after another-7&8)
Your decrease for the neck edge will always be on a WS row and at the far end of it like it was on row 1 when you did the first neck decrease.
The underarm is easy just every other row at the far end where the big BO was. On the neck edge do the first decrease, then count 3 rows (I like to make hash marks on paper) and do the decrease on the next row (the 4th).