1. I have had swapped grannies fall apart on me. I have also had swapped grannies made incorrectly, with 2 or 4 dc per side, and not enough ending tail to fix. I would much rather weave in my own tails.
2. I agree crocheting over the starting tail works best.
3.I tend to make double the "required" grannies to keep for myself, so upping the ante wouldn't be a problem.
4. I use a ton of variegated yarn, and the matching solids. Adding the option would be a great idea.