After everything he's done, Rerir does not deserve to be happy or to have some cookie cutter peaceful conclusion. BUT, I do have a deep pity for both him and Thlondis and where they both ended up.

Regardless of the corrupting effect of the Abyss, and how he was effectively manipulated down the path he was on (by it AND Irmin), Rerir was still conscious to and cognizant of everything he was doing. Him show genuine regret and remorse — after the fog of the Abyss was lifted — was perfect, but all the Abyss did was draw out and amplify his worst qualities. It was still his decision to walk down the paths he did.

Ultimately, he did not deserve to be redeemed and (thankfully) he wasn't. She's been dead for centuries and, now, she finally knows everything; there was never any chance of the two returning to a normal life, no matter what the power of the Abyss made him think.

He will forever spend his time knowing that Thlondis is so very close by, but that they will never meet again. In addition, he has to sit with the knowledge that it was all for nothing, not because they will never be reunited (and he will never hear her answer) but because he was fighting so hard for something impossible.

Thlondis being both gone and right there (literally within reach but unreachable) and the pair knowing that their actions and lies are the backbone of what will forever keep them apart — even with the pair's lies finally revealed — is a deeply and appropriately poetic end.
[Luna IV Spoil] Rerir's End Feels Appropriate