I was really surprised how much difference even the 10mm tube made on a 35mm lens. I am not sure how far I’m going to ultimately take this because I think I want a longer focal length (yay flighty bugs) and the amount of tube needed would be a bit excessive. I had OK luck with my 150-500, which offers a minimum focal distance of around 23" and a magnification of 0.32 at the wide end. Sadly, zooming pushes the minimum focal distance out.
Both. Ethonal is still corrosive and the majority of fuel systems these days are compatible with E15. That said, check your owners manual.