This is a wonderful summery for everyone who is reluctant to try out pattern testing and the testing is so important for the designer.
I guess in Europe (and on ravelry) the graded patterns are more common so for these I would add one item to your list of how designers choose the testers which is important: the size/figure of the tester. For longer there is a movement to make more size inclusive patterns in crochet or knitting and I noticed that pattern testers with more unusual figures (very small or big sizes) are in higher demand than people between size XS-L.

So far I only once applied to be a tester and then had to decline when I was chosen, as the designer wanted the testers to use exactly one kind of yarn and this was so expensive - wow, I would never buy something like this. I found it sad that we could not substitute the yarn…