a) $400/25= 16 buy-ins. If you have money you can afford to reload then it doesn't matter but if it's your set bankroll then I'd move down to $10NL (40 buy-ins) for now while you work on your game. I can't comment on whether you should be playing LHE. It's a different game.

b) Don't be results oriented. Work on getting game and results will come.

c) Yes, playing full-ring is highly recommended for beginners. It's slower and therefore easier to multitable and lower variance than SHNL. Don't add more tables because it's slower though, stick with 4 for now and work on getting better reads.