I don't think the Redskins are nearly as bad as you seem to think. The defensive front 7 is strong (and the return of Jarvis Jenkins, who was viewed as their top DL guy, will help). They need a CB and a FS (more competition than anything, not that they need an elite guy in there at FS, as they have some options), but they have the makings of a solid defense. Offensively (besides QB), there's some raw pieces. They need two OL (RT and interior ... assuming Trent lays off the drugs) and a top WR, but they have RB options and a dynamic TE.
At the end of the day, if you truly believe that a QB is elite, the trade is okay.
That said, yesterday I cautioned to watch out for Tannehill and the Skins, and today, the Post noted the same thing. His skillset fits what the Shanahan's want. I think they probably have RG3 as their top option, but if the price gets too high, Tannehill at 6 may be appealing.