PLA is totally fine, I print all the ones I sell in a high-modulus carbon fiber-reinforced PLA. If you're concerned about strength print with more perimeters (3 at least) and 0.2mm layer height. The only concern would be extreme heat; PLA isn't nearly as heat resistive as other thermoplastics, but you'd have to get to uncomfortable temperatures for that to be a problem
The 'tassle-looking things' are Peak Design Anchors. They are a system that allows quick release and easy changing of straps. They're aimed at photographers but their straps work equally well for a lot of different uses.
I think the guitar strap idea is a bit cumbersome and bulky. I might not be understanding the intended mechanics correctly either, but I don't think it will work as intended. The connection point (I think they're called 'buttons' ?) of a guitar strap would probably come loose since the way the controller hangs and the mounting points of the controller are totally different.
