Maxwell, I'm assuming in your comments below you are referencing DAW: Battles and not the simple combat system in DAW: Campaigns.
1. While ranged fire doesn't cause drive backs directly, light troops can avoid casualties from ranged fire by voluntarily retreating. So in practice, during the exchange of missile fire, you have a choice of clearing out the enemy's skirmishers without doing lasting harm, or of attriting the heavy troops. It works very well. Driving back heavy troops with ranged fire doesn't work because the movement rates are too slow. Compare the DBA shooting range v. spears movement rate, versus the ACKS shooting range and phalanx infantry movement rate, and you'll instantly see what I mean.
2. Great question - you've hit upon the #1 problem area that I've had with DAW: Battles. The key problem is that in 1 combat round, a PC can cast 1 spell. Therefore, if you have battle rounds in DAW: Battles that are equal 2 or more ACKS-scale combat rounds, you grossly power-up magic. For example, a "wand of fireballs" in 1-minute battle rounds could deliver 6 fireballs. Deadly.
There are several potential solutions to this problem:
a) Have a long battle round, and hand wave magic. The DAW battle rounds represent 1 minute or some other length of time, but only 1 spellcasting per character per round is still the rule. This approach doesn't work well because it badly disassociates the battles from the RPG. For example, an invisible mage who gets into position on the flank of an enemy unit suddenly finds he's...spellcasting...in...slow...motion....
b) Have a long battle round, and allow lots of magic. For a substantial length of development, DAW used this approach. The mechanic called for an I-go/you-go approach to magic during a separate magic phase. If Side A had a mage with a wand of fireballs, and Side B had no mages, Side A could just fire, and fire, and fire. This was realistic, but grossly unfun, as it made the battles decided by whomever brought more magic to the table.
c) Have short battle rounds, equal in length to 1 combat round. This is the approach I've settled on. I'll discuss why below.
First off, long battle rounds are not inherently more realistic than short battle rounds. That is, it doesn't *inherently* take 100 men a minute to cross 100 yards. It doesn't *inherently* take 100 men a minute to stab people. Long combat rounds are just meant to model friction and chaos and "slow things down" so that battles take a historically reasonable amount of time to resolve. Military historians hotly debate how long the "actual battle" took. Did forces in phalanx class for hours, or minutes? They can't agree. DAW, with its short battle rounds, takes the position that most of the time in a battle takes place during deployment (pre-battle) and pursuit (post-battle), while actually fighting is short and intense. So that's the theoretical basis for short combat rounds.
Second, short battle rounds allow for a 1:1 correspondence of RPG action and DAW action. At any moment, you can "zoom in" on the characters, and resolve their actions, without disrupting the flow of the mass combat. This is a *HUGE* advantage, especially when you have higher level PCs with lots of tricks up their sleeve. It also means you can integrate, e.g., the mortal wounds table, encounter movement, and so on, with ease.
Third, short battle rounds put mages on a balanced footing with regard to other characters. Since ACKS is balanced around 10 second combat rounds, using similar length combat rounds keeps the mages where they are "meant to be".
As far as fighter-types, the most important thing they can do is kill off enemy leaders and commanders. Killing off enemy commanders grinds the enemy's command and control down, causing their units to have worse initiative, less flexibility, and more brittleness. Actually causing casualties is secondary. That said, a high-level fighter with a couple henchmen with magical augmentation can certainly deal 50HD worth of casualties.
3. I've mostly answered this above. It's hotly debated and I don't fault anyone who prefers to imagine ancient battles as long grinds. In actual play, it tends to take 4-6 rounds to grind away an enemy heavy infantry unit, so the "scrum" takes about a minute. This is roughly what Victor Davis Hanson suggests was common in "The Western Way of War", which is shorter than what Delbruck and others thing. I'm agnostic as to what it "really" was like, but the short rounds work great for the game.