The Isle of Mists quest has each segment ticked and shown in green.
The moment I reached this destination, the point of no return, the following entry
was in red with a cross " speak to friends to help fight the wild hunt"
This was part of Brothers in Arms quest which I completed, however Seira Mentz
was not available as instead of my sending her to Kaer Morhen, I let her go to Radovid.
She is no more part of the game so how the game can say I failed the quest beats me!

You failed the quest. Keira Metz (not Seira Mentz) is a friend of Geralt and you sent her to her death.

Thanks for the correction. Actually I did not send her to her death. I let her decide and she decided to go
to Radovid feeling that he would spare her life. (can't remember her reason for this)

(can't remember her reason for this) - She thinks that Radovid will spare her life if she give him the pox cure(the manuscripts from tower)...

Yes, you did send her to her death. You (the player) made a decision not to help her, to let her take her chances with a madman who tortures and executes mages without exception. You (the player) knew this; knew the consequences and still let her go, so in essence you sent her to her death.

OK, so I sent her to her death. (debatable)

Are the "without exception" and "knew the consequences" things necessarily true at this point in the game though? Radovid's madness unfolded over the course of the game, depending on how soon you traveled to Novigrad, saw his handiwork, got the details from Triss, and interacted with the man himself. It's perfectly possible to get to this decision point with Keira in the game without seeing firsthand any of that, or even hearing any major accounts of it. Geralt has a lot to say about how it's suicide during this dialogue but Keira flatly contradicts him with all of her responses.

Trying to think back to a first playthrough, I can't recall the choice feeling anything like an option to send her to her death. Geralt's "sending" choices are both conversation enders, that signify him just walking away. "Do what you want," and "Do what you think is right." I always felt the "oops now she's dead" result was supposed to blindside you, like a punch in the gut, as a unforeseen consequence of a seemingly innocuous decision to leave someone to their own devices. Of course, I did "For the Advancement of Learning" very early on.

======================
Regardless, Keira's a shit friend. "Strictly selfish" by her own admission, she manipulates, betrays, and drugs Geralt, and after showing up at Kaer Morhen, says she wishes she hadn't come at all. She has utter contempt for Geralt's relationship with Yennefer ("she treats you like a dog" ...pot, meet kettle). My first playthrough resulted in a fight, not intentionally, though I should have guessed she wouldn't react well to "Can't let you do that."

Your sentiments regarding Radovid, I agree entirely.

Thanks. Now that I recall your previous posts, Hickory does have a point though. In this post you said you've played through game at least twice already. Surely you must have known that letting Keira go to Radovid would result her death.

Precisely. That is why I stated in parenthesis "you (the player)". This was a known, avoidable outcome. Meta gaming, sure, but still the player's fault in this case.

As I sent her to Kaer Morhen in my last play through I decided on a different approach this time not realizing the consequences of this i.e she is no longer available for alchemy purchases and Lambert dies at the Kaer Morhen. battle.

***SPOILER***




Lambert doesn't die there. If she's not available, you have to jump in and save him. Her appearance just means one less fight you have to participate in during that sequence.

If it was not Lambert then it was the other witcher. I can't remember his name right now. I was running along the pathway when someone shouted that has been killed.

***SPOILER***
Vesemir's death is unavoidable during the Battle of Kaer Morhen. Lambert's is not. If Keira is at Kaer Morhen she automatically helps Lambert when he gets in trouble with the Wild Hunt. If she's not there, Geralt needs to get to Lambert in time to save him. If he does, Lambert lives, if he doesn't Lambert dies. Nobody else can possibly die during the battle.