AC vs DC Measurement using an Oscilloscope of a half-wave rectifier
I'm going to call the voltage that you're measuring on the scope \$V_{in}\$.

When you set your oscilloscope to AC coupling, the voltage that you see on the screen is:
$$
    V_{in,AC} = V_{in} - \text{average}(V_{in})
$$
so turning on AC coupling effectively just shifts the signal down so it's centered at 0 volts. 

If you're looking for the actual value of \$V_{in}\$, AC coupling isn't a great way to do it - there's no way to tell what the average value was before we removed it.

You asked a lot of questions, so I'm not sure if I've answered everything.