2 - We can not trust the CV. Maybe the company did a test, maybe they didn’t. Maybe their test are to our standard, maybe they aren’t. Maybe the candidate barely passed. Maybe the candidate actually failed but made up in other ways that might be relevant to the other company, but not ours. I doubt anyone would give us this information about their current or former employees. Maybe the candidate really did work on the projects he claimed or maybe he was only tangentially involved. That is a lot of unknowns. A test clears up what the person actually can do.
3- Yes. It works for us. Do we miss out on some good candidates because of this? Yes, but it definitely prevented us from hiring bad candidates.
Again: if there is a better way then I would like to know. I have not found one yet.