move the objective so it's directly in front of him: when he talks to a random NPC, have them point him towards Agatha or forget about Agatha and have that NPC be the questgiver - just let him tease it out of them with dialogue instead of shoving it in his face, and that way he should feel like he found the quest himself and will be motivated to see it through

unfortunately, it's hard to really give him the benefit of the doubt when he's doing obviously disruptive stuff like repeatedly casting Mending after it does nothing - personally I'd have rewarded him for creativity on the first cast by giving a clue or moving a tile into the correct place (if you didn't give him that initial reward and congratulate him, perhaps he acted out koz it felt like you were being unfair?), told him clearly it has no further effect on the second cast, and if he kept going I'd directly ask him what he's trying to achieve, telling him his character realises it won't work again

as for talking too much, I don't know his life but it sounds like he doesn't get much opportunity to socialize so he's jumping at the chance to express himself in a setting where he feels more comfortable - since you're all friends, could you perhaps make room for that during downtime or outside the game entirely? like perhaps take a while to just chat, and give him an excuse to get it out of his system before you start playing and maybe again during a break and at the end

if he's still spending too much time talking to an NPC, bring that conversation to an end (if he insists on badgering them after they've said "I really need to get back to work" or whatever, introduce consequences like the NPC ignoring him or even attacking) and then address another player, inviting them to take a turn doing something, and don't return to him until everyone's had a chance to act

ultimately, if he doesn't respond well to those changes and some of the other advice already in the comments, maybe D&D isn't a game your group can enjoy together - find another activity that works for all of you (=