The Deathless Ones burying Kitezh under ice to keep it safe, and in the process, burying thousands of citizens and enemies alike, either alive or crushed by the ice. Keeping it safe for them to stay immortal, is more like.
It's revealed in one of the various documents in Kitezh proper that a lone survivor recorded how he "killed" a Deathless One by caving his skull in and the Deathless One bursts into smoldering ash and embers. Later on, the survivor awakes to find the Deathless One whole and unharmed, and ready to go again. The survivor does all this, risking what precious time he has left alive and knowing he won't be able to report it back, to learn about his enemy.
It's elevated even further by his realization that the Deathless One he killed remembered him.
The followers of the Prophet fleeing to Russia from Constantinople is a strange vector until you remember that the Rus were also Eastern Orthodox Christians. It's also a place which is extremely far from Trinity's control, since the Catholic Church had no sway there.
It seems odd at first that Lara can somehow learn Medieval Greek, Modern Russian and 12th century Mongolian by looking at various murals, scripts, and artifacts - some of which have no samples at all of the relevant language. But when Lara first tries to read Medieval Greek, she admits she is "a little rusty". She's not unfamiliar with these, she's reminding herself what she used to know, likely using the various texts she finds to rebuild her familiarity. The same could also be true for her knowledge of Russian and her Mongolian.
Why does Atlas feel so entitled to Croft Manor? He's obviously a very old-fashioned aristocrat with an interest in maintaining traditions, hence his willingness to perpetuate antiquated practices (i.e., arranged marriages, over-emphasis on family honor). He probably thinks the house is rightfully his because Richard Croft never had a male heir, and he's next in line. Of course he's never going to admit to that since it would be bad for his public image, so he's instead looking for any legal precedent to use as an excuse.
