My questions are:
1. Is my army list viable? What would YOU change?Yes it is viable. Nº of characters depend of nº of brigades. As a guide, General y heros could lead 2 brigades each, and wizard could lead 1 brigade. So you could add a hero or swap a wizard for a hero.
But being warmaster a game of tactic movement, having characters gives more choice of fullfil the tactical plan you elaborate.
Your list is strong in powder, that is cool against chaos, but remember shooting units do not win the battle alone.
2. How useful are Wizards really compared with another halberdier unit?Wizards could lead brigades with no decisive roles (move artillery, move reserve units, gather disbanded units). And could help with lighting bolts in the shoot phase (offensive role). Stop the charge of a single enemy unit (defensive role), made a unit of your army move twice (in some moments this is really good, command role), and move instantly to any part of the table (non very interesting role).
Halberdier unit is the backbone of your army, the way you rise your breakpoint and the bricks of your "defensive wall". Another brick on the wall is always wanted.
If you have enough halberdiers to make a solid "wall", then take the wizard. If you are lacking of halberdiers or low breakpoint, then go for a halberdier.
3. How many heroes incl. general is needed / optimal for 3000 pts.?see1. But there are no optimal configuration. Depend if you play offenisvely (lots of characters) / defensive (few characters).
4. How do you use brigades most optimal in an Empire army?There are a lot of configuration, and not always are optimal for all the players. Depend of the way you lead your army.
But try to:
Artilery brigade: protect your cannons
Hard hitting brigade: use flagellant in the first row, they will not be stopped by shooting
Hard hitting brigade: gatther some knights
backbone of the army brigade: 2 halberdiers+2 crossbowman
5. How do you field and protect cannons / steam tanks? Do you gather the cannons near each other or spread them out and do you use particular units to protect them?Depend of the way you are playing.
You could gather cannons and concentrate fire to blow your enemy up.
You could see the cannons as element to stop the enemy advance, making "forbiden corridors" for your enemies by the menace of be shooted. This way you could control (more or less) the movement of your enemy. Spreading the cannons could be interesting in this way.
Xbowmen are cheap and works fine as artillery bodyguards
6. How best to use a General on a Griffon (do's and don'ts)?General could fly and move from side to side of the battle quickly to give the orders where are needed: to flank withe the cavalry, or to approach those cannons to hit the enemy flank, or gather inf brigades making a wall of sharp halberdiers, and so on.
If you are a gambler player and the chaos dragon+kingts starts to make really hurt, then you could try to enter in a balance combat and negate the fear of the dragon with your griffon. This is risky, but is also very brave and spectacular (but ... who wants to live forever, and die old upon a lonely throne

)
7. How best to break a chaos line?Some people should say artillery and shooting. The idea is good but is difficult (not imposible) to stop chaos with shooting.
Use terrain. The way of defeating the chaos furius attack is using wisely the terrain and defending position and also being bold to attack them. I mean not stay the whole game in defending position. Warmaster is a game which prizes the "attack player". Empire player could not be the attack player in every turn, but need to attack if you want to win the battle.
8. How to best combat the many fast movers?With more men and authority. I mean fast movers usually isolate themselves trying to flank or moving fast to attack some position. If you have more units than the fast attack enemy, and they protect each others (making big brigades, in example) your men could envelope the isolated fast movers.
To sum up, keep a coherence body of your army with defined battlelines. This way fast movers could harm you but not rip your army.
hope the answers helps or at least have fun with them