Are you locked in matching gem slots?

That is partially correct, but not complete.

Arcane Barrage should generally only be used:

A) When moving (and Presence of Mind is on cooldown - for an instant cast Arcane Blast) and if you know that your Arcane Blast debuff will wear off before you can cast another one to keep it rolling (to not lose the extra damage modifier).

B) Arcane Missiles (not AB, as you put in your post) can be used 'on proc' generally once you have 2PC Tier 10 bonus as the use of the Missile Barrage proc gives a 5 second haste buff (Pushing the Limit)

For example, when I run as arcane (less often now that I am primarily fire) I generally run an ABSpam0234Mbam 'rotation/priority'.

Quick question, is haste pot or wild magic pot best for fire?

Depending on your level of haste, and other CD's being used at the same time...it depends. If your GCD capped, the haste pot is less valuable, or not at all.

The quickest way to find out is to use Rawr, go to Item Budget and check your relative stat values. Let X be your spell power coefficient, Y be your critical strike rating coefficient and Z be your haste rating coefficient. Then:

If 200*X + 200*Y >= 500*Z use Potion of Wild Magic, otherwise use Potion of Speed.

It will almost always be the case that Potion of Speed will yield higher benefit, unless you have very low spell power and critical strike rating, combined with high haste rating. Or in extreme cases when you have some sort of haste buff (think General Vezzax) and a Potion of Speed will bring your Fireball's cast time below the GCD.

If clearcasting procs while a Fireball is casting and the next spell cast is an instant spell, are both spells mana free? Similiar to how Sindragosa's unchained magic only stacks once using the same mechanic.

this one?

#showtooltip

/cast [nomodifier:ctrl] Living Bomb

/cast [modifier:ctrl, target=focus] Living Bomb

/stopwatch 12

/script Stopwatch_Play();

Keep in mind the timer won't start counting down the first time you use it when you log on, unless you /sw beforehand.

I just tested it now. It shares 3-min CD with healthstones, but not with mana gems, the same as on live.

I tried it the way you described it and Potion of Speed is indeed superior when calculated in such a way. The way I usually check it is by going to the Options tab -> Spells & Buffs, there you can tick or untick the potions and see the dps difference directly. When done in such a way, Wild Magic is superior for me.

Yep I pretty much agree with this. Crit is only slightly superior to haste which is why 500 > 400 most of the time. However, I have seen a lot of discussions about speed vs wild magic under herorism/bloodlust. Some ppl argue that prepot with speed and use wild magic under herorism is better. Does anyone know more about that issue?

Since it was nerfed to only proc off frost/fire ward, mana shield and ice barrier (and not, for example, PW:S) it has become much more situational. It is still of some value on certain fights where frost or fire damage is prevalent, but points in MA are probably of more general value.

For a full discussion on the current state of IA see this thread (which is old enough not to show on the default view).