Would take too much energy to be worthwhile.

Seriously, think of it - you need to put in enough energy to lift a weight with the mass of mercury several hundred thousand kilometres. Ouch.

Indeed.

Plus, what would we get out of it? Any of the "bonuses" that Venus would receive are still useless because Venus is a smoldering hell... Even if this crash managed to clean up Venus' thick atmosphere, it's still too close to the sun... Heck, even if this crash moved it out a bit further, it'd still be too close to the sun...

As for moving Mercury, though, since it is not a static object, I don't think you'd need to put in all of the energy required to move it the necessary distance... You should be able to just "nudge" it, and let its own momentum push it in the new direction... Simply redirect the energy it's already using...