Hmm, having a go-along NPC might work, but I am not sure. How would they know the party wouldn't overpower the NPC if it decided to get into mischief, start an evil cult, rob a temple, etc? I think it would work better if the go-along or the temple had something upon the fighter to prevent him from such behavior - a Mark of Justice, perhaps? Or maybe a curst item that only the Bishop or his minion (the NPC) could remove. It would not need to be major - just enough to inconvenience the Fighter most of the time, but with a potential major effect that could occur on command if the commanding one was within a certain distance.
Say something like: Bracers of Steadfast Striking of the _______ Temple
These sacred bracers will only affix themselves to the arms of a character if a cleric of the ______ temple aids in placing them upon the character. Once affixed they grant a +2 sacred bonus to attack and damage rolls.
However, they are charged with a Geas / Quest spell, and each morning they must be blest by a cleric of the ______ temple or the benefit is lost and the penalty of breaking a Quest spell apply (3d6 damage and automatically sickened: -2 to attack rolls, weapon damage rolls, saving rolls, skill checks, and ability checks for 24 hours). If a blessing by a cleric of _____ temple is placed upon the bracers the penalties are removed and the benefits regained. However, the benefits are lost and the penalties are gained also should the character wearing the bracers ever attack a member of the clergy of _____ temple or deface / desecrate any temple or holy item of the _____ temple.
Once put on they cannot be taken off except by the cleric that put them on or a higher ranking member of the _____ temple. Also, there exists a command word that will immediately bring about the penalties (and remove the benefits) of the bracers. This word is usually set shortly before putting the bracers on (and usually out of the hearing range of the one to soon wear them). Only those knowing the command word can make use of this - often the NPC who put them on and a local higher ranking cleric who oversaw the placing of the bracers upon the character's arms. The effects of the use of this command word only work once in any given 24 hour period. The word cannot be used to increase the penalties with additional uses.
If the temple had a control / leash such as this, I can see them allowing the Fighter to leave - supervised. I furthermore would expect the bracers to allow the Bishop to easily scry the fighter.
As for the quest - who says that all holy avengers must be long swords? Perhaps the bastard sword version was a unique creation that was lost, and the Bishop is asking the Fighter to seek it out and return it to the temple. Of course, perhaps the item, after being captured, was corrupted and granted sentience - a NE bastard sword that perhaps can mimic the powers of a Holy Avenger - or has powers the reverse of a holy avenger: an Unholy Avenger, seeking either a paladin to corrupt to its service or a blackguard to aid it in seeking out and slaying the paladins that once used it. It can hide its nature, seeming little more than an enhanced bastard sword (like a holy avenger while not in the hands of a paladin). The bishop knows of this and either wishes to corrupt the Fighter to his service or at least to gain the weapon - so that it can be granted to a more 'worthy' candidate - who would become the bodyguard of the bishop.
Either way, I still think it would work best if the temple / religion at large did not realize that the Fighter had been freed. If the Fighter is later captured, the bishop can either say the Fighter is lying or admit to having released him - and point out the powerful means he had taken to contain the possible menace should his [the bishop's] mercy had been misplaced. That way he could still come across as good - just a little misguided / mistaken due to his 'rosy-hued' vision.
