[Spoiler] Humans on earth

[Omnitool]

I do recall a lot of pictures or computer terminals that implied there were many more underwater facilities in the Pathos global network

Can you post the screenshots then? Because I do not recall a single one. Gameonly mentions PATHOS-II, there was nothing about PATHOS-I. Additionally, the numerous characters in the game (audio logs + Catherine) keep repeating "they are indeed the last humans on earth".

There is this map of all the underwater bases, mainly owned by the corporations:
http://tinypic.com/view.php?pic=i2vybm&s=8
It is found in the game files, I don't know whether it is found in-game, I don't remember seeing it...

Also, how the hell did the surface structure at the end of the cannon (Omega) survive the giant tsunamis that resulted after the comet hit the ocean, it would have been literally dragged all the way to Greenland?!?
It seems like a weak floating platform, it can't have possibly survived the forces that literally tore the entire planet apart! The entire length of the cannon also should have been bent like a needle struck by a hammer given the tsunamis, tectonic shifts and continental displacement!
It is very unlikely that both the entire cannon "barrel" and the floating end-point Omega platform are in a practically intact state, still fit for launching projectiles, while the base of the canon has been clearly damaged, with water pouring in the launch control room (where the Pilot Seat is located).
In comparision if a sniper gun's barrel would be bent even by 2-3 milimertes, the projectile would shatter inside it! How on earth did the cannon's barrel not bend even an inch when an entire planet was ruined is beyond my understanding.