Ah, I thought the decrease just below the blue circle was the decrease for the underarm. What is that earlier decrease? Do you have an underarm bind off on the other edge of the sweater?
You’re right, because it’s rib and seed stitch you can reverse the front. Right side or wrong side, it’s fine either way.
To attach a new strand of yarn for the second shoulder, leave about a 6" tail and begin knitting with the new end of yarn. The first stitch or two will be loose but you can snug up the tail to tighten the sts after a couple of rows.
For the neck, you’ll pick up sts in some places like the decrease area along the side of the shoulder and you’ll knit some sts directly from the holder.
All your questions are good ones. You’re doing really well and the sweater is going to be very pretty.