Congrats in finishing your first helmetliner! I like to do them. I think I've done seven now. I understand about the black yarn. I did my first in black. Tan is so much easier for me.
I use Cascade 220 & Galway. Both work for me. I make the neck ribbing 7 3/4 inches long and still have yarn left over. For the above the eyes part, I use a provisional cast on cause I don't like picking up stitches. I usually do a Russian bind off. Is stretchy. I don't worry about the gap beside the eyes. It helps the fabric to not be stressed when the part that covers the mouth is below the chin. On the first one I made, I knew the recipient. He just tucked the gapping fabric in.
I hope all goes well in your surgery.