So! Update regarding the levin sword and mechanics regarding it when it comes to Robin; protips for Robin players here.
There are two ways to use the Levin Sword in the air while you have it equipped; first is by smash attacking in the air, and the second is by already holding it in your hand prior (you were partly right, Godot), if you used it for a smash attack on the ground or already used it in the air. If you want to switch back to the bronze sword in the air, either perform a neutral air or a tilt/jab on the ground with it, which'll make you be holding it again; this'll make your tilt air attacks become the bronze sword again until you use one of the methods to use the Levin Sword in the air again. Of course, if you've thrown away the Levin Sword temporarily due to being out of uses, you'll be using the bronze sword in the air even if you smash in the air.
Shame the circle pad decides to think your tilt is a smash on occasion.
As a disclaimer, it's still really hard for me to confirm this; when I switch back to the bronze sword, it still sometimes turns Levin on me even if I only tilt in the air. This shit is complicated.