Not sure exactly how to pull apart the stand or unscrew the arms … but nice chairs 
I used to work on Alum group chairs - at an old workplace, a weld in the base would break and a couple times a year a broken chair would get tossed. I’d fish them out of the dumpsters and re-weld them. I’ve definitely taken the arms off before with those screws, and I think it’s just lock tite they originally used.
In your case, it seems you could either force it, possibly damaging the threads or carefully put in a spacer to take up the gap.
One other suggestion, Just in case you might want them as perfect as you can get and try polishing them - there is a protective layer on the mirrored aluminum that gets scuffed up over the years. If you remove the layer to polish out any imperfections in the metal, make sure you clearcoat it again right away - that aluminum tarnishes really fast!