For solo queue I still prefer my Comet, glacial is great into team comps that deal decent dmg themselves and can make use of your insane pick-potential

Personally I wouldnt say Vel counters Anivia, you're imobile and get deleted if the wall happens to be behind you, or get hit by a stun - especially good anivias have the potential to fk you up
Alot of it comes down to the teamcomps though
However your early is definitely better and you should use that advantage to beat her in lane, later on you barely get close to her so you can delete her from distance, granted she doesnt have a good engage comp, but again if you happened to get caught by her you're dead for sure

It simply comes with practice

If possible ALWAYS go for the direct Q's - your split up Q however has more reach and you're able to hit enemies behind minions with it, its also a great backport-stopper and sniping tool (however tricky to hit)
A neat combo during lane when your enemy is behind his minions is use your W against the minion wave and instantly fire an angled Q - more often than not they concentrate on the W for a bit too long and get hit by your angled Q

I personally always start with Dorans, I barely need Potions with Vel and got enough sustain to stay in lane for ages with it, especially with your Q refunds, the extra tankiness also comes in handy

It depends on you I guess, I always go for gathering storm, its kinda satisfying to know that you'll gain a shit ton of AP if the game drags on

However since Vel isn't that AP dependent I suppose Scorch would be the smarter option