After the Battle of Kaer Morhen, Keira Metz remains ALONE around Kaer Morhen, on the hill, where they buried Vesemir. And she is always there. And this despite the fact that the story inserting explained that Keira went with Lambert in search of adventure.
After all, what kind of nonsense happens?

После битвы в Каэр Морхене, Кейра Мец остаётся ОДНА около Каэр Морхена, на холме, где хоронили Весемира. И она стоит там всегда. И это при том, что сюжетная вставка объяснила, что Кейра уехала вместе с Ламбертом на поиски приключений.
В итоге, что это за бред получается?

Yes it's a bit strange but what confuses me more is that the door that would lead outside to her is closed.

The Moor has done his duty, the Moor can go. I guess just one of those "gamey" oversights nobody cares for.

У меня только мат в голове по этой теме. Ничем не могу помочь.

There's still a way to get to her, you just have to climb something if I recall correctly which allows you to bypass the locked door - I did it a few days ago.

if you complete another quest in the main story line & actually progress the story all the NPC's will be updated & in Keira's case will leave Kaer Morhen doors pfft! shes a sorceress why use doors when you can summon a portal

I did that too but it's a bit annoying

And why those mages in Novigrad couldn't teleport away? /grin

this could be explained in numerous ways.
Summoning a portal to teleport one person is difficult enough as Geralt finds out at various points in the game, disassembling numerous amounts of people & reassembling them in the right way in the right place would take excessive amounts of power which could have devastating effects on its caster. Not everybody being saved is a mage or sorceress, yes they may know magic but the majority of them haven't studied magic so lack the combined power or knowledge to summon a portal. Triss is a powerful sorceress & could possibly teleport everybody but the risk just isn't worth it, also teleporting leaves traces which can be by those who know how(Witch Hunters) be tracked

As far as I remember lore, there is no such thing as "Weak mage". There is either a mage, or not a mage. So teleporting should not be an issue for anyone who is mage. Herbalists and alchemists are different case, that's correct. But mages shouldn't have major issues getting out of the city. Maybe even getting back with surgical precision, Yen, Keira, Triss, they all do teleport, and Yen teleports to great distances.
Traces are dispelled through gameplay - there is at least one case when there is a teleport inside Novigrad. Short range, true, but under noses of witch hunters. Also there is been teleport to Novigrad or Novigrad's vicinity.
Mages escaping to Kovir is still quite an interesting thing. If Radovid is alive, and he is powerful enough to beat the crap out of nilfgaardians, what will stop him from invading Kovir just to cut mages? Kovir is not that big or powerful, or remote to be entirely secure. So even if mages left no trace, I doubt news of their new home will be secret for long.
Also, Triss being wanted, yet she rides in that ball dress from Novigrad to that vengelbundelschmundel estate and back with another endangered person, whose existence already known to witch hunters, who even laid an ambush for him. What is that, bad unit cohesion? Triss doesn't exactly look like dirty laundress, yet, apparently, she had no issues riding through on horse (not teleport; and if there was teleport, then it was short range).
tl;dr While I can find "feasible" explanation, this moment is where I can't suspend the disbelief. "Because reasons". :)

I've completed the storyline, I've got the Lambert + Keira Romance ending, but she's still near Kear morhen, alone. No Lambert around.

You know Lambert's difficult character. So she teleports to places Lambert (and many players) can' t reach.

So it's nothing else than an omission by CD Projekt.
It's kind of sad to see Kaer Mohren empty by the end of the story, but "c'est la vie", I guess.

Wasn't Kovir actually richest and most powerful of northern kingdoms?

It was once an impoverished region until the massive mineral deposits were discovered. Now it is probably the richest, but definitely not the most powerful.