Is that the only logical answer?
The Strokes had to delay the release of a single once due to complications with the cover art.
I think the whole not touring thing is really the only logical answer to why there isn't a single yet, and its a pretty good reason! If they don't need to push a single out sooner because of tour shows, it would be better from a marketing perspective to wait until the album release is closer, that way the hype around the single doesn't die down before the record is released.
There is no reason to think there are any problems because there isn't a single yet, its all good!
It would be quite logical that RR releases something just before preorder starts, they don't assume that everyone will preorder just because it's DT (otherwise what's the point in promoting?). It doesn't have to be a single, it could be a snippet. I would think that the hype rising after new material surfaces would increase preorders.
RR may want to release something when pre-orders start but its not needed. If they release the single after pre-orders start it will still generate hype and people will still have the opportunity to pre-order up until the album comes out.