Believe it or not, Rom's mechanical aptitude wasn't a retcon.

Quark's comment about Rom having no mechanical ability was him voicing his traditional low opinion of his brother, and the truth as he knew it at the time. I seem to remember that a few episodes after that Rom shows mechanical aptitude (I think it was during a break-in) that puts Quark to shame. Plus, Quark is totally floored that Rom not only knew what he was talking about, but that Rom could actually do it.

So even if they decided at the last minute to reverse course on Rom's character and make him an engineer idiot savant, they gave a pretty good in-show reason for it and covered their butts in a pretty realistic way.