The only time "if able" could possibly apply is at the moment you are gaining the target lock. Let me paraphrase the alternative point of view, to ensure I understand it correctly.
1 - Kagi requires you to target him "if able".
2 - If you were performing a target lock action, Kagi would be a legal target (he is within Range 3)
3 - Therefore, your ship is able to TL Kagi in the general sense, which means he is now the target of your TL.
My issue with this is that if you require Kagi to be the target based on what your ship is generally able to accomplish, rather than restricting it to only what your ship can accomplish right now, then Kagi could and would be the target of target locks no matter where he was on the table. It can be argued that even if Kagi were beyond range 3, it would be possible under other circumstances to TL him, so he must be the target. Your ship is still "able" to target him generally speaking, even though right at this moment he is out of range. If you do it this way, the "if able" phrase on Kagi is meaningless, and the rules for TL are meaningless as well since Kagi's ability would completely override all restrictions.
It seems to me that the only way to apply restrictions is at the moment you are attempting to perform the action. If you are attempting to perform a TL, Kagi must be the target if he is legal right now, under the circumstances you are currently attempting to perform the TL under. Whether or not he would be legal under other circumstances is immaterial. You are not able to target lock Kagi if he is not a legal target right now, regardless of whether that is because he is out of range, or because you already have a TL on Kagi, or because Fire Control System does not allow Kagi to be the target of your Target Lock.
Kagi does not remove restrictions, or make him a legal target when he otherwise would be. This is the key component of the argument against FCS providing a TL on Kagi when he wasn't the target. Other wording is immaterial, you have no legal mechanism available to target Kagi. It doesn't matter whether or not he is the required target, because you cannot target him at all.
An analogy would be Biggs. If Biggs is out of range or arc, it doesn't make any difference what Biggs' ability says because you don't have the ability to shoot him. Nothing in Biggs' ability allows your weapon to shoot beyond Range 3, so if he is out of range you simply cannot shoot him and may shoot other targets freely. The perfect example would be this:
Your HWK has gunner and an ion turret. Biggs is at range 1, but not in your primary arc. When you fire, you fire your turret at Biggs because he requires you to do. According to the FAQ, you do not have the option of choosing a different weapon, you are required to use the weapon that is capable of hitting Biggs so you MUST use your ion turret, and you must target Biggs with it. However, if you miss and Gunner activates, your next attack will NOT be at Biggs no matter what else is going on because Gunner only activates the primary weapon. The fact that you "could have" targeted Biggs with your ion turret is immaterial, you cannot target him right now, using the weapon you are using right now, because Gunner does not allow you any attack you wish it only provides a primary weapon attack. Gunner can not activate your ion turret, so you don't have the choice. Biggs' ability does not override the fact that you cannot use your ion cannon at the moment. IF you could, then Biggs would work, but you CANNOT, so Biggs' ability does not work.
IF you can target Kagi for a lock, right now, then you must. But if you cannot, then Kagi's ability is immaterial. He is NOT a legal target right now. Nothing in his ability changes that essential fact, regardless of whether he would have been a legal target at some other point in the turn or under other circumstances, just like Biggs' ability does not override the restrictions of Gunner regardless of whether Biggs would have been a legal target at some other point in the turn or under other circumstances.
Edited by KineticOperator