That would be good but, unfortunately, it would make little sense. After all, if the Empire had already been there at the time of Rogue One they would've known immediately that Leia was lying about the location of the rebel base (there would've been no need to send scouts/probes).

A trailer for a trailer?

Rogue One International Trailer. Not a whole lot of new stuff

No, it's the scientist played by Mads, whose character is wanted by both sides.

Mads is playing the guy who is responsible for creating the death star. Calling it now.

The geonosians might not have designed the weapon itself.

Yes I remember that. They might have designed the space station but not the weapon itself. Lots of parts has to go into the death star, I suspect.