(makes it a bit sloppy, but not so much the iron falls out). It just floats in air rather than making contact with metal. The downside of course, is it's more expensive.
It looks like there's a slot where the wire goes. Maybe it's adjustable, or at least removable and replaceable with a better fitting widget.
There is a slot. The wire makes a U shape (squared at the corners), which fits into said slot and is inserted all the way down. I don't see any advantage of inverting it, or only partially inserted (not bottomed out).
That said, it would be possible to drill out the plastic to 1/2" if you need it to be larger. But as we're talking about T12/T15 tip cartridges, the existing size would work very well. I say this as they're similar in diameter to the WMP I use, which has never fallen out on me.
For the record, I own a WDH-10, WDH-20, and WDH-20T. The differences between the basic 10 & 20 models is the tip holders; everything else is identical. The T variant is the switched version to get full setback features.
FWIW, it is really easy to make a T12 iron holder that doesn't wiggle, at all. The diameter of the handpiece is exactly 3/4".
3/4" forstner bit, 5/8" bit, a piece of hardwood board standard 3/4" thick, a piece of thin sheet metal. Drill the 3/4" hole a 1/2" deep. Drill out the rest of the way with the 5/8" bit. Chamfer the opening and sand/ream out the 3/4" hole until it's snug as you want. Cut a V notch in the sheet metal and screw it to the back. Adjust it so that the iron is perfectly perpendicular and straight, handle tucked in the 3/4" hole, and the sheet metal preventing the iron from drooping. Same idea as the wire on the expensive holder, but you can remove all wiggle up/down and sidewards.
Certainly possible to DIY something.
I would suggest making the base weighted so it's less likely to fall over, as well as provision for either a sponge or brass wool (IIRC, the 599B is 2" in diameter).