Jesus christ does this dude become your biggest hater if you free him, he straight-up must've invaded me like 6-8 dang times across the game and refused to give up despite getting his *** trashed every single time he Invaded me, Man's must be my Number 1 hater for sure, followed by Mr./Ms. Forlorn

Why does he persistently try to invade and murder you as thanks for freeing him? Ungrateful much? Jokes aside, any specific reason why he does this besides him being a lunatic?

Ser Aymeric?

You can complete his questline in NG after defeating Throne Watcher & Defender, Nashandra, and Aldia.
Once the credits have (eventually) finished you are taken back to the Majula bonfire still in NG.
Jump off the cliff to kill yourself if you are human, retrieve your souls, go to the mansion in Majula and talk to Cale to get his helm, then go to Aldia's keep and complete the quest (assuming you have Gilligan's Ladder Miniature, The Sunset Staff, and the Aged Feather).
You do not lose any of these items to Navlaan, you just have to show them to him.

Mindlessly freed him in a run where I was rocking an INT build without realising he was the NPC that gave you Unleash Magic lmao. Oh well, there's always the next NG+

Brador is that you?

If you pulled the lever and he refuses to trade you, you can burn a bonfire ascetic at the bonfire right outside the building when you walked in and he will trade you.

I was chaotic and released him then killed him immediately after. He didn't even have a chance to fight back.

Ran into a glitch where Navlaan had his fog gate released and he wasn't there when I first entered Aldia's Keep.

Navlaan Deal, Navlan Wheel.....wait...wrong npc