OK, let's see.
EZ may not have knit a steek. I don't have the fearlessness of EZ, so I knit steeks. I am making a sweater now where I'll have to sew and cut without a steek since I didn't know where to start it--the sleeves are knit separately and then you cut for them to fit. I'll take pictures of that potential fiasco.
I like having the security of the knitted steek since I don't have a whole lot of confidence with the sewing machine--the steek stitches give me a little leeway.
I don't sew down my steek edges. They just kind of lay there with the machine stitching still in place. I don't see how they could unravel after picking up the sleeves, and I have had no problem with them. I'll take a picture and show you what it looks like. It might be neater to sew down the edges, but I'm not all that particular how my sweaters look on the inside. I don't really like to sew for appearance, so I don't.
The way you shape a neck with steek stitches in place is to put the center stitches on a holder, then cast on the steek stitches above it. The shaping is done on the outside edges of the steek. So when you decrease, you do it outside the steek and edge stitches. The steek stays the same size, but the stitches of the sweater on the outside of the steek go down in number.
I'm in the process of making a cardigan, and I have cut the steek up the front (I just have to finish the bottom of the sleeves--it's been waiting since last summer.) I just looked at it, and the neck shaping was done the same way. Keep the steek as is and decrease on the outside edges of it.
I hope that answers your questions so far, and I will get some pictures of the insides where the steek stitches are for the arms and neck.