Some games and apps have files outside their install locations or OS-specific dependencies; that is the reason for the limitation on archives created from existing installs. If you use the Download Archive option on a machine without the game currently installed, it will get the original package that is not OS-specific (These can also be gotten via Impulse Anywhere).

Try grabbing the package from here: http://anywhere.impulsedriven.com/

Yes.

From another thread I was reading a while ago, I got the impression that it was actually practical to keep an up-to-date archived version of the game, in case impulse ever became unavailable.

So basically from a future-proofing perspective, archiving isn't completely useless, it is just completely unwieldy because you have to remember to archive before you install, and completely impractical because you have to download the entire game again every time it patches.

At least it is still useful if you format your PC using the same OS...