Fails to crank problems are really frustrating. Been there, done that. The good thing about them is that they are usually fairly easy to diagnose and fix. There’s about a half dozen common reasons for that problem, that’s the main challenge, determining which of the 6 it is. If you feel lucky you could just replace the battery and starter motor again. This time use a VW unit from a dealership. That method often works and avoids the diagnosis component.
Yes. This is called a voltage drop test. Google this.
Had the exact same problem with my BMW 740. My mechanic diagnosed it right away as a bad starter. He replaced it with a new one and no more problems.
You can bench test the starter. Remove it and place it on a solid surface. Hook up leads to the started and ONE to the battery. Take the other lead and touch the other pole of the battery and observe the started/solenoid. You don’t need to let it run for long, a couple of seconds is good enough. The pinion gear should extend and rotate. If it doesn’t you have a bad starter.
There is another way, maybe not 100% reliable but it might work. Have someone start the engine while you (or your wife 
) is under the car with a hammer. As the person is attempting to start the engine and you hear the click click click give the solenoid one or more solid “taps” with the hammer. If the engine starts you have a bad starter or solenoid for sure. Make sure you are far away from moving parts or you could get seriously hurt.
If you can see the started from above you can do it using a metal rod or some other tool to “tap” the starter.