How to invisibly join superwash singles yarn?
Superwash single is a somewhat tough one, yes.

In those case, I usually start ten or so stitches before I change the yarn, I use the same technique as when doing jacquard to have the new yarn follow in the back of the work, I knit one stitch with both yarns, and knitting with the new yarn, I have the old yarn follow the stitches on the back for like ten stitches.
On the next row, I try to have the loose yarn follow on the back, so that it holds more.