@ Charel

The pushback negation from Icy Veins is only in effect as long as Icy Veins is active. That means it’s only good for 20 seconds every 3 minutes.
Not everyone takes points in Arcane Stability, in certain fights it's very nice to have and in others you don't need it at all.
I don't know whether Ice Shards affects Blizzard or not, it would stand to reason that it does.

A priest that raids with me keeps advising me that I should press Arcane Barrage just as the fifth missile of Arcane Missiles ticks as it will also gain the Arcane Blast debuff, however I have never managed to replicate this effect myself.
Is this true and I am just too slow, or have they been misinformed?

Misinformed. It worked like that for like 2 weeks at the start of patch... 3.2 I think? But it was fixed shortly after.

This was a bug that was fixed a long time ago. It hasn't been possible since Naxx.