length of zipper for a down bag

I think you'll find that trying to add a draft tube on, after the fact, will be a big problem. What is reasonable and necessary is a snagless fabric strip alongside the new zipper. If you don't insert that, you will curse yourself every time the zipper snags.

–B.G.–

Make it a quilt!!!

But to answer your questions

A draft tube is needed unless you put the zipper low enough that it will be under you side, but not the most comfy solution. You can use an anti snag backing or you can sew a few lines of stitching through the draft tube to keep the fabric tight to avoid snags, yes it does take some loft from the tube but shouldn't have a real impact on temp rating

I would suggest zipper to the knees or a bit lower, i have converted bags for people with 1/2zip that they hated because they couldn't vent their legs/feet (I converted them to quilts)

-Tim

The reason I don't make a quilt is the draft factor from being a side sleeper. And yes I have made and tried a quilt before (a RJ style), the draft coming in around by back was very uncomfortable especially near 5c.

I will take your advise a make the zipper don to my knee area.

The draft tube will be a separate tube filled with down, with a crossgrain backer against the zipper to prevent snags.