Maybe it would've been nice for the Stone to heal Garrett of his blindness. But his lack of vision doesn't hinder Garrett, and it doesn't stop Kayley from loving him. Also, from a writing standpoint, it probably would've sent the message that disabled people can't adjust and live happily.
Why Ruber's Villain Song looks more like a mediocre poem. The guy is completely and utterly insane, so of course he wouldn't be able to construct a proper song.
Devon and Cornwall get a tiny example — maybe — following their weird duet about wanting to be separate. Garret reluctantly allows them to come along on the condition that there be "No singing!" Devon inquires how Garret feels "about interpretive dance." The man is blind. The question mark for the Fridge Brilliance lies in whether or not Devon was aware of Garret's disability at the time.
Wonder why everything is alive in the Forbidden Forest? Celtic Mythology is famously animistic.
It seems strange that King Arthur of Pendragon would knight someone as obviously unhinged and selfish as Ruber, indicating that he's a Horrible Judge of Character. But then you remember how wartorn England is implied to have been ten years before the movie, and say what you want about Ruber but he's certainly an exceptionally powerful and determined warrior. It's not unlikely that he actually did play a large part in Arthur's success at conquering and bringing peace to England, which would also lend some justification to his feelings that he deserves a proportionally large share of the rewards. Add to that that insanity tends to escalate over time, and it's possible Ruber was a very different man before being cooped up for ten to twenty years in a peaceful and stable country that no longer suited his brutal politics or way of life.
Ruber mentioning he bought his potion from some Witches makes sense, when you remember there are Witches in Arthurian Legend like Morgana Le Fay, Nimue and Morgause. Many stories feature them having a rivalry or enmity with Arthur, so they would be willing to help Ruber conquer Camelot.
