Will a CTS-V MESH grille fit an 05 STS?

no two different sizes n shapes

Check with Luke (Lindsay Cadillac Parts) on pricing for an STS grille that will fit your car.
As noted the CTS grille is a very different shape and they are not interchangeable.

Yes it will............with a lot of modification.

Yes, you can teach a dog to talk. If you start early enough (gentically speaking).

I don't think you could make it look right. The proportions of the STS and CTS are too different. CTS-V grill would look too small...