Your standards are probably too high. I have the Big 4 DVD, and Dave sounds like I'd expect him to sound live, that is, I expect him to sound bad live because I've heard many people say he sounds bad live. And he does sound bad, but I can still hear his voice, and that him doing it in the studio a bunch of times, he can get it right. Not playing a guitar at the same time is also probably a huge help.
As for chords, I thought you wrote them all down?
C
D
G
Am
Em
Those are the most important chords, As a metal guy learning power chords is also a good idea. Those are easy, you learn one, you know them all.
Power chords are three notes, those are always the root, the fifth, and the octave. A "D" power chord is D,A,D, for example. But you don't need to know all the notes on the guitar to know a power chord, because the shape is always the same. So your "D" power chord will look like this:
e---------------------------------
B---------------------------------
G--------------------------------
D--------------12-----------------
A------------- 12-----------------
E--------------10-----------------
Move that same shape down to the 3rd fret, for example, and you've got a "G" power chord. This works anywhere on the 6th or 5th string, except on the open fret, but since you know A and E, you already know how those chords work. On the fourth string, you can do the shape with the 3rd and the 5th, but the octave is going to be one fret up on the second string. So a D power chord starting on the third string would look like this:
e-------------------------------
B--------------15----------------
G-------------14-----------------
D-------------12------------------
A-------------------------------
E-------------------------------
So that is the other power chord shape. That version is less common though, most of the time guitarists will just use the root and the 5th for that string. But yea, most commonly used chord shape in rock/metal. Maiden uses it constantly.
I hope this makes sense.