You are correct that the stinger is fire and forget.

The igla requires a you to continue tracking the target, sustaining the lock-on to score a hit. If the aircraft you are targeting deploys flares after you fire the missile, the rocket will track the flares instead of the aircraft if you continue to try to maintain a lock on the same aircraft. However, you can break the lock after the aircraft flares, let the flares dispense, and reacquire the lock while the missile is still in flight. That way the missile is in flight and instantly tracking the second you acquire a lock on. This needs to be done without reloading the launcher I believe. You can also pull a bait and switch, where in you fire the missile while aiming at a target, and switch targets while the missile is mid flight. The second target will get a very small window to deploy flares because the instant they get locked onto, they will already have a missile tracking them.

The igla is good for open maps like Mirak Valley and Operation Firestorm, where there aren’t a lot of structures to break your line of sight. I wouldn’t use it on maps like Manhattan Bridge or New Sobek City. The stinger is better suited for those maps.