I make a cut down the back of the neck, starting at the base of the skull and then invert everything. Once you are ready to close it up, I sew it closed about half way, then insert the skull back inside. By doing about half of the sewing while it is still inverted it makes keeping feathers out of the seam alot easier. (Note: I only use the skull plate and not the entire skull so the entire skull won't fit if you try sewing it half shut first). I would NOT try the gluing the seam shut until you have MANY birds under your belt and still wouldn't chance an oops myself. Since it only takes maybe a minute or two (tops) to sew it closed why chance making a major issue ? The number one thing to remember when sewing the seam closed is to NOT pull tight. I leave it VERY
loose and slack. On ducks, sewing is only to hold the skin in place until it dries so a tight stitch isn't ever needed (IMO) If you pull the head and neck tight, the feathers will end up being a preening nightmare. In most cases making it impossible to get them smooth. By leaving it loose you still have the ability to adjust where as being tight or glued leaves you little to no flexibility.
From the appearance of your buffie's head it looks like you pulled it tight because of the way the feathers look to be fighting against each other along the rear of the head/neck juncture. You did a dam find job on a first bird so be proud. I only wish my first attempt had looked like a duck when I finished