Mirrors are not effective enough. Shielding drones from energy weapons seems like a similar problem to entering Earth’s atmosphere, you want to shield it in a way that will blast away safely and ideally diffuse the laser, so the energy is spread over a larger space. I suspect larger lasers will likely aways win, since there is only so much shielding can do. At which point we could end up with transformers like drones that are built to be broken apart mid flight and yet still deliver damage. I feel like defending drones could become possible with energy weapons but only under ideal weather conditions.

Likely cheaper to just coat the real drones in an aerogel or similar light weight, high thermal resistance material. It's an arms race still, but one with a reasonable amount of asymmetry in favour of an attacker.

I'm not certain, but I think the returned beam would likely be significantly out-of-focus.

No, but an AI drone like the one Turkey has can probably detect the source of the beam by hiding behind some sacrificial/decoy drones and watching them blow up then shooting a missile at the laser source. It's not like the laser is coming out of thin air.