1) Can be made in the round, just turn all purl rows into knit rows. If you use the cast on in #2, there is no sewing!
2) I usually cast on with a variation of the provisional cast on, but just using the one yarn - hold yarn like the long tailed cast on, put first loop on, next loops like provisional cast on. This allows you to pull the cast on part tight, like a drawstring.
3) Ball can be stuffed with plastic bags (recycling), or with a mixture of polyester fluff and cedar shavings (like they sell for small animal cages - stuffing with only cedar shavings makes it too lumpy). The cedar shavings allow the ornament to be tucked into a drawer, or hung on a sweater hanger, to make clothes smell nice - a good quick multi-use gift.
4) Egg shapes can be made by making the ornament longer.
5) This is a great way to try out novelty stitches or yarns. What I do is look through all my stitch guides, and pick out some colorful ones that I have wanted to try, but haven't fit into any projects. Then I adjust the ornament size to the stitch repeat needed for the pattern - you get 3 times the cast on stitches for the middle part of the ornament. For example,
Cast on: End up with in middle: 8 stitches 24 stitches
9 27
11 33
12 36, etc.
For numbers between these amounts (say, for a pattern that wants a multiple of 3 plus 1, so you are aiming for 34 stitches), cast on 11 and just do an extra make one in the 5th row, or cast on 12 and leave out 2 make ones. Be careful if you are knitting in the round, but trying to follow a flat knitted pattern.