Hi there, with regard to the circle left behind, yes I do get that but a lot fainter than yours. Mine disappears after a few days. Just as well really or we'd all be looking like we'd got some strange skin complaint after a few months. x
That looks a little sore. I've personally never had an issue with the Libre leaving a mark? (Apart from the little hole.)
Regarding keeping the Knightrider in place. There is a company called "Guardian?" That make flexible laser printed re-useable armband holder things?
I use a Miaomiao 2, but I have seen them for your transmitter too.
It's a bit "Freddy Mercury" with the band, but I find mine quite comfortable with activity which also protects the Libre as well?
They also come in a variety of colours.