The number of sts per repeat is noted in the directions for each pattern st. Scallop edge sts, no change in st number; Granny st, the number changes as noted in each row and summarized below the pattern directions. You can use markers (loops of yarn will do)to keep track of st number and see where you're going wrong in each repeat. If you're off by so many sts, you'll need to decrease to ~72sts or the neck will be huge.
If you like the look of the lacy poncho, then finish by knitting the other 3 sections. I agree that binding off and picking up sts for the neck is more work. That's why the pattern suggests using stitch holders. You can simply slip the sts onto a spare length of yarn if you like. The neck band is straightforward knitting.