1. A good control scheme lets you rotate with one hand and move left-right with the other. Less than a handful of top players have been able to make it without having that balance. You can find the controls that everyone uses and why here: http://harddrop.com/forums/index.php?showtopic=2983
And here is a smaller thread on the topic: http://harddrop.com/forums/index.php?showtopic=2749
I personally spent a lot of time trying everything I could and ended up with this, which made me considerably faster after a couple weeks:
a: soft drop
s: left rotate
d: right rotate
f: hard drop
j: move left
k: move right
To summarize what you will find in that larger thread on controls, the consensus is that space bar should not be used for hard drop. I like to use my index finger since it's faster than a thumb. Also, all of my keys are on the center typing row so they are easy to find (just feel for the j and f grooves. But most importantly, left and right rotate are next to each other (unlike default) and are on a separate hand than my left-right. If you've been reading through these forums at all I'm sure you've noticed the high frequency of recommendations to rotate with both directions to improve your speed/finesse.
2. I have asked this question several times and no one really knew the answer, but the consensus I gathered was that they need to be the same value. The intuitive explanation, which Kitaru and Wojtek still have not corrected me on (game developers), is that having different values will create a window for when your DAS will fire off rather than make it consistent. You do not want to risk that because you want your DAS to be predictable. What 7 MIN DAS literally means is that 7 frames will go by after holding down your left or right key before it starts to slide to the side. So a max DAS that's higher implies that it can take longer for it to kick in, which would hurt your feel of the game.
3. ARr is Auto Repeat rate. Most of us have it on 0; I know Blink uses 1. That's how fast your piece starts sliding once your DAS kicks in. On the nullpo menu, it's called DAS Delay. I am yet to see anyone who uses 1 AR account for why they like this delay, although one of our players, aaron, says he likes to add 1 AR for multiplayer and he does pretty well. I still prefer 0 for multiplayer (as do most).
4. Disclaimer: Hard Drop is not affiliated with Tetris Online Poland. But... it is probably the best. All the upgrades are free, soft drop is almost instant, it's less laggy than TF, and there is no line clear delay. Content edited out.
5. I've never changed the music in nullpo, I just keep it on silent and listen in the background, but if you want to change the files this looks like a promising place to do it: Nullpomino folder > musiclisteditor.bat
As far as nullpomino settings, one that I recommend in general options is next display type: side (big). That makes it easiest (for me) to see the piece preview. I would also turn down the sound effects considerable, and certainly put an X after line clear animations (I think everyone can agree on that last part).