I don't think it's specified whether it's voluntary or not. There was mention of one guy having trouble adjusting to his new name, but that doesn't necessarily mean he still couldn't have taken it by choice.

No, there are no other ones named after philosophers (or real people at all) that I can recall. I think LKH just decided "eh, it's Ancient Greek, whatever!" and tossed it in. Ditto for how she also probably went "close enough!" with the Roman myth name of Bacchus.