How can I cleanly finish and starts rows, using single crochet, in my pokeballs?
Genrally in amigurumi you don't crochet in rows, but in spiral. This way there is no visible seam at all, but you usually need a stitch marker to keep track of where rows begin and end.

Here is a photo of a bottom of a bunny I made, to illustrate how it's done and looks: