I should have looked at your images before hand I guess. if you were making a full circle, than you'd want some wiggle room. Seeing your not doing complete circles, I'd think modeling it at 20mm would work. WSF is a porous nylon material.
I'll go back to your original questions
Will some materials tension wear out when under tension over time?
For your application, I don't see that as a problem. For thin walls, and lots of flexing it probably would wear quicker
How flexible is flexible as in WSF? Like rubber?
It's more like Polypropylene, but more resilient. if you bend a PP part it'll crease, where WSF would be harder to permanently deform.
Are there already similar applications i missed?
I think most the iphone cases use the flexibility of the materials to snap onto the edges of the phones. This includes the detail materials.