Sorry, I was in a hurry. I don't like the idea of a duel defined as a soakfest, so I will make it OHK with multiple rounds. Different strategies, but this is the way I will say it. Also, I would say the area you stated would not be suited for duels of even water guns in general. The obstacles are too far apart.
Anyway, in situation one, if I had the Vindicator, I would utalize shot time and range to supress the FF and keep him back, avoiding the dangerous point blank situation. I would keep on smaller nozzles to try to outlast him as well.
With the FF, I would move quickly, never standing still. A flanking maneuver would be helpfu, as my speed would be faster, and I could force the enemy into a corner. I would also climb on the obstacles to negate the range advantage of the Vindicator, and at the same time, crouch, reducing my profile.
Stream Speed: FF?
In situation 2, if I had the 2500, I would utalize the length of the gun to reach out my gun and get more range. I would rely on the 10x for maximum range and clarity of the shot. I would also go for the long range confrantations, and keep the enemy in sight.
If I had the 3000, I would stick to 5x or 10x to gain superior shot time over the enemy. I would push for short range engagements, and dare the 2500 user to shoot up their PC. I would also use my gigantic capacity to outlast the enemy.
Steam Speed: Tie?
In situation 3, if I had the Vanquisher(CPS), I would press the enemy to get the enemy into a battle of long shots, due to my larger reservoir and PC. I would also avoid short range battling, due to the awkwardness of my gun.
If I had the Equalizer, I would stay on the move, and try to get the enemy to underestimate me. I would also try for close battles, as my smaller gun can corner better.
Steam Speed: Vanquisher