never seen what a diagonal legs while curling looks like, I wish u had a video example. I'm only familiar with a bent knees, I think it prevents cheating as well as it's just appropriate posture. I feel as if the "stroke" of the barbell is "shorter" from the bottom to the "squeeze", make reps and sets a bit smoother.
I do not wear gloves anymore. I did, and I think that may have contributed to some wrist issues sometimes. Keep your wrists straight and do not cheat with your wrists to get more squeeze. LOL. Or get some wrist support strap thing.