Why does the HAV allow the driver to operate and fire the main cannon at the same time?

Perhaps an addition to satisfy both HAV operators and those who feel HAV operation should be a team effort would be to have a driver seat like the LAV.

A single operator would be completely protected as they currently are, but would have to switch seats to a turret to fire a weapon. Multiple operators would mean a dedicated driver and manned turrets, while solo drivers would have to stop moving and switch controls to attack a target. Retreat would then also mean switching back.

I believe this may offer a good dynamic.

If tanks are AV focused - What the hell are they for?
That literally means that they are pointless outside of their own microcosmos. You wouldn't even need to call in another tank when the enemy gets one, because an enemy with a tank just means that one of their squad members suddenly stopped contributing meaningfully. That's why large AI turrets need to exist.

With infantry, I can be either AI or AV. As a commando even both. Let's disregard the imbalances that exist for the current AV situation.

I don't think you currently understand the advantage infantry have by lone-operated HAVs.

Currently, to see where you are driving, an HAV must have their turret pointed in the general direction. This allows people to sneak up behind HAVs and place REs, or AV nade them. Give HAV dual operation and you have a constantly mobile heavy weapon platform where turret tracking speed (which is relatively slow) does not hinder the ability to retreat.

I've had many HAV's screw themselves over because they panic and they reverse without swiveling the turret before hand and thus run themselves up a rock and they lose the vehicle. Allowing the HAV turret operator to be given only a single focus would be bad because then we'd have HAV operators controlling their vehicles much more easily. The HAV driver only has to worry about movement and HAV health while the HAV turret operator only has to focus on targets. Currently, the HAV operator has to focus on the HAV health, the targets, and where they are going. It would be better to leave as is so we don't lessen the load of responsibilities handed to a HAV.

No I am not a tanker I do the shady sands shuffle.

You are currently able to hunt tanks because tanks are good at killing infantry. Where do you think all those people that are risking their HAVs are coming from? Those are not NPCs. They do not spawn from thin air. They exist because they feel a reason to exist and risk their ISK.

Tanks are already restricted in how they contribute. Their only role is killing/area denial. Spawning and scanning is better done with dropships and LAVs. They currently appear in massive numbers because they are basically invulnerable and can easily slaughter infantry, not because they contribute to the match in any other way. I've seen matches lost because the team was tank driving instead of capturing objectives.

Now you are saying that they also lose that role and instead require a two man support team in order to kill infantry, which is the real support role due to the fact that infantry is the basis of the game. It also creates the unique situation that any useful tanks will be good at both killing infantry and tanks alike, because your AI turrets have to be manned in order to contribute to the team.
And good luck winning a match where six blueberries are driving two tanks.

Big turrets = AV means that tanks are only called in when there's already another tank on the field. But why the hell would that happen, especially with the turrets that are placed everywhere? In this vision, there wouldn't be tankers killing off turrets at the beginning of the match, which would actually weaken tanks in general and make their AV focus even more pointless. Seeing a tank would boil down to one of three situations - Blueberries that want to try them out and don't realize that they suck at AI, call in and instant recall of a railgun tank against a dropship or another HAV and a dedicated three-man crew that wants to farm infantry kills. You will not have sole tank operators anymore.
Allowing big turrets to have different roles allows tanks to be real support plattforms with specified niches. One guy with expensive equipment that has dedicated counters.
Balance the game around infantry, not around tank battles that don't have any reason to exist without infantry interaction.

And, dude, make up your mind. Your first half of the post is about customization of tanks for different roles. Your second half is about having all big turrets as AV. I can agree with one but not the other.

I would vote for the opposite approach:
Allow solo dropship pilots to use their guns in some way. LAVs can already run people over when run solo and are fast enough to evade attacks.

HAV solo turret operation is completely okay as long as it's fixed on a single target type.
Make the turret good at either infantry OR tank killing. And then force them to use additional gunners to attack the other target type.
This means that a tank as omnipotent as the current one would need three operators, but doesn't make them useless when run solo.