Voadam
Legend
Right, human thieves and bards in 2e can explicitly wear elven chain type armor and get better ACs than leather armor provides. Bards (who can wear normal chain too) suffer fewer thief skill penalties when wearing elven chain than chain mail. Elven chain mail also can fit under clothes so you can wear it in settings wear visible armor would not be appropriate. So there are reasons for humans to want human sized-elven chain even if they cannot cast wizard and bard spells in it.Yeah, but then there's 10% of Elven Chainmail which is sized for humans. And there's the question of half-elves as well.
As far as dual class humans this seems to be addressed generically too on page 62 "the character must still abide by the restrictions of whichever class he is using at the moment. A dual-class fighter/mage, for example, cannot cast spells while wearing armor."
As far as half-elves I am not sure about how you are suggesting this muddies whether non-elves can cast in elven chain. I would think either they are classified as elves and can use it, or are not and cannot. That classification call may be unclear, but once it is made the application of the elven chain rule seems clear.
I don't think those things make it less clear.So it is quite a bit less clear than it first appears.