I wrote to Elsevier to ask them about their involvement with the arms trade. Their response is below (and as PDF here, 600 KB). They only answered the first of my three questions (with a ‘no’).
Frankly, just because something is legal doesn’t make it legitimate and anyway I find hard to believe that adequate checks are carried out at DSEi, especially given that we know it has, just for a first example, repeatedly haboured the brokering of illegal sales of landmines. I’ll be writing back to Elsevier, and in a few days I’ll post it that here too.