Translation:Gentlemen and ladies, welcome to the conference!
I agree. I think this is the case even in English. 'Ladies and gentlemen' sounds better than 'Gentlemen and ladies', as 'ladies' is shorter, but I also think 'Men and women' sounds better than 'Women and men' as, once again, 'men' is shorter.
"Ladies and Gentlemen" is accepted by Duo as correct here, presumably because, as others have said, that is the normal English way of saying it.
Gesinjoroj is what I tend to hear that avoids the need for gender order. I do wonder if Sinjoro or sinjorino mean anything now that landed gentry isn't much of a thing. Does it really mean that the speaker simply respects that person, rather than that person owning land?