Thanks for the input and the drawing too!
Yeah, the bouncing is wack. Originally the idea was, just like in your drawing, where the tip of the sword hits the mirror, the center of the sword will be placed there as it goes on its merry way, effectively being predictable and nice and rainbows and everything, but I couldn't make it, the mirror bounce was enough of a headache to make, took a lot of time as well, mainly because it wasnt just a simple "rotate the vector 90 degrees" as I'd hoped for, so when the actual mirror bouncing needed to be changed, and it was all buggy, I just threw my hands in the air as I was already running out of time. Another option that was on the table before this free-positioning of the mirror is having a grid based system. Which would have ALSO solved this issue, but having never used tilemaps before, in the interest of time this was the best I managed to do.
Sure, they deserved it.... absolutely. No question about that.
edit: Also the current implementation DID use a big box collider like the one you drew, which was absolutely horrible, the current one used a small sliver of a capsule collider, which as you saw still far from perfect. Thinking about it now a simple raycast may have been better but oh well