Thanks @Ahti for the update - I'll mention your comment in the guide!

I dont play online to create offline duplicate and i cant find the save files (they are not in the places people calim where they should be)

Anyway, I managed to complete it
1) get the Wilhelmina rumor
2) find Bass and accept his quest BUT do not tell him about Wilhelmina yet
3) complete Bass quests
4) get Bass to talk about the suspect
5) after that tell him about Wilhelmina (it is the separate dialogue that we ignored before), this should give you his trust
6) go to Wilhelmina and tell the truth about Bass, gaining her trust

@Ahti - I created this guide while I was playing the game, I would have to replay the game as I have deleted my backup saves to check the conversation options.

I also can't test this as I already have the achievement. You can try it by exporting a character file before you choose the conversation options so you don't have to replay the game. And I can update the guide if you know the conversation options.

Could you please give the exact conversation options and order for the Double-Tongued?
I'm not going to play this for the third time just because of one achievement

This is quite literally the only thing I could find related to the realms rebuilt update in order to complete some of these. So thank you so much. I tried really hard to google varying phrases and nothing turned up, so it was nice to come here and find something. Cheers mate

You're welcome, its refreshing to see a good guide once in a time. And I'm also happy to contribute :)
Yeh, I guess you're right with the "missable" definition here.
I'd just say its a bit about point of view. For me, as I played before we had internet connection (SNES Nintendo, Playstation...), its something else than for those novadays teenagers, running 'n shooting, fancy only about the graphics.
Fun fuct about Nightingale: you could, in extreme, mark as missable some main quests. I noticed that some sites are accessable even without finishing quests (but obviously, I didn't try tu run them thru).
For example, you can simply fly & climb up the headquarters in Sylvan's Cradle. And if you climb the observatory in Welkin's reach, you can jump right in.
Oh and, by the time I write this, you haven't edited the info at the top of your guide (about Oisin & Nic).

@squalx Thanks! I've updated the guide :)

I'd say the others are still missable, in the sense that if you aren't following a guide and get to the end of a playthrough they are most likely missed and your character is locked from completing

Hey mate, nice guide.
Just completed all the achievements (before I read this), and I can confirm you can get
both Through a Glass Darkly and The Devil You Know.
Simply don't tell Nicolas anything at first, later promise Oisin you won't tell, then betray him to Nicolas. You can then even go back to Oisin and tell him you betrayed his secret, for a nice reproaching dialogue.
Whenever I started a realm, I looked to not miss anything, checking all the achievements as I progressed the questline. I don't know how accurate Steam datas are, but I got like half of the sidequest achievements while they were on 0% global.
That said, I would not mark any achievement as truly "missable" except Act of Defiance (obviously) and Balm and Bitterness.
About Balm and Bitterness, you might note that it become available only if you read all the echoes in the observatory. I almost missed that one because I missed one note. I had to go back as I read through the guidebook and noticed they were incomplete.

@Rekalty - thanks, I've added a note in to that achievement!

Might be an inaccuracy regarding the "In Grove or Green" achievement. I got it from doing the Sombre Site of Power, not the Radiant one. Did the Radiant first and didn't get the achievement, so either you just have to do the Sombre one, or you need to do both.