As for how the Klingons maintained range, they always won initiative. Once the BB went down (in that particular game) they had a +2 to the roll, and had more ships at that point. By using F5s as initiative sinks and having the initiative they were able to make us move a ship, then move to within that magic 3" band. If you note, we were able to close with 1/3 of the Klingon formation, but by the time we flew through the 24" and 12" disrupter range bands while using 1/3 to 1/2 of our facing phasers for drone defense (we were out of ADDS by turn 2 in most cases) there wasn't much left.
So its not that klingons maintain that 3" band. But that some ships are in it. Well that is always going to happen no matter who you fight, just as you'll always get ships in your best range. What is also going to happen is that you will lose ships in a fleet game. Accept that and move on. If he moves to 4" to kill you with Ph2s then he takes damage if you explode. It is a lot easier to get Ph1s in range 8 than Ph2s in range 4", plus they hit more often, and at really close ranges you will be hitting weaker rear shields with a number of ships, so your damage goes further as well, not to mention you have very good phaser banks with good arcs.
I can't understand how you can be out of ADD at this point (closing to range 7-15 on turn 2). At range 18"+ there are so few drones hitting that a few phasers and shield boost negates them. Even a C8 and 2 C7s only puts 4 drones on target a turn on average, hardly enough to warrant risking ADD when you have plenty of phasers (that you don't need for shooting at that range), tractors and a shield boost. You shouldn't have rolled a single ADD at this point, barring some major luck on the enemy dice rolls.
You do have ranges you are better than klingons at. When photons are up then 0-15" is your better range. At 12-15 you have a major advantage in ship killing ability, even at closer ranges you are a lot better than him. Longer range is easier to do and will happen sooner usually giving you an early lead. He may get to fire next turn when you are reloading, but he is just making up for the damage you already did in one fell swoop.
At range 15" 16 photons have about a ~20% chance of killing a D7 and ~20% chance of crippling it, so ~40% chance of effectively rendering it ineffective. Depending on points/ship selection a 12 ship fleet may be getting 3 such volleys in.
It takes about 44 disrupters to achieve a similar 40% figure against a Fed CA, and even then that is mainly cripple chance and only a small chance of a kill. Even at ranges less than 12 it takes about 35 disrupters to do what 16 long range photons do.
At long range you also out phaser him, accentuating your heavy weapon advantage. Throwing in Phasers with the photons more than doubles your kill chance (depending on how many Phasers, I'm assuming 6 phasers per 4 photons). However, at long range many Fed phaser banks are barely affected by klingon shielding, so it may be worth just shooting at a different target with them. If you are using phasers in banks of 2 then you only lose ~10% damage at long range. Concentrating photons on 2 targets will often kill/cripple those targets, and mass Ph1s will take out another target. Someone above said take out the smaller klingon ships, which is a valid tactic as it gives you movement advantages in future turns.
The main point though is that at long range you are barely affected by klingon shields. Fixating on how to bypass klingon shields and agility via flanking early in the battle is just wrong. Your weaponry already counters his shields and agility. Certainly photons are streaky, they are in the source game as well. But if you are rolling bad with photons then Klingon's having tough shields is rather moot.
If you are finding it awkward closing on klingons then ACTA has done a good job of representing the SFU - certainly in FC a fleet level game would often see the Feds down a ship or 3 before they got anywhere near photon range. In FC a 12 ship Klingon fleet can cripple a ship at range 25* and kill one at range 15. They have the agility to probably maintain range long enough to take out (kill/cripple) 2 or 3 ships before facing serious return fire. Klingon drones were an issue as the Feds were the ones having to close through them to avoid getting hammered at long range.
* I should add that is without scouts. That would be very unusual in my experience where pretty much all games involving more than about 6 or so ships have included scouts since they became part of the standard rules. Scouts help mitigate the long range fire, they also gut the photons as well though. I haven't played with a scout in ACTA yet.