Go to the timeless isle and grab the 5% stat buff and the Chi Ji buff, use the voidwalker to tank for you and practice tracking and reacting to your trinkets and cooldowns. You should be able to fill in all the holes in your gear and get to 500 in a couple hours. You will be able to much more clearly see the effects of hitting your CBs with trinket/skill procs because the numbers show up much higher. You'll also be able to practice proper havoc and shadowburn usage. It's worth running LFR or grabbing a normal pug via oQueue with a fistful of Mogu Runes (1k timeless coins each) a couple times to go for Wushoolay's off of council (or LFR for Garrosh's similar, superior version) so you can have very significant spellpower values to practice reacting to (this will shorten your learning curve significantly).
Destro plays a bit like a warrior where you generate rage and then spend the heck out of it with colossus smash up- you do pitiful damage if you don't properly manage your colossus smash, and destro does pitiful damage without proper ember management (I'm seeing some 530 warlocks in flex doing 60k). Needtoknow is good enough, but there are a few other addons that can work significantly better (for example, you can make a weakaura that tells you your current spellpower/haste/mastery/crit ).
In terms of grimoires/pets, many top-5 WoL parses are done with the Fel Imp, it's much better at target switching than the Observer, and cauterize master is a not insignificant heal on a 30 second cooldown, especially on hardmodes when your healers are undergeared for the content. Until you really know what you're doing with Destruction, I'd highly recommend using a pet. Sacrifice is fantastic for experienced locks who know how to get the most out of ember generation and use. If you're still very new and people are counting on you to perform well, it's not a great idea to lose that safety net in favor of theoretical min-maxing.
The difference is very small. Observer is the best only on Parchwerk-like fights, and most of the fight aren't like this.
Imps are ranged attackers and does more damage when you need to change your target often. i.e. Nazgrim.
Imp can also dispel or heal you. When dps is sufficient while other problems occur, it is useful. Just imagine the situation where you need mass dispel but all priests in your team are absent.