Wait till you land and fight on Horizon....

Yeah, I noticed that was much tougher than most other parts of the game, even only on veteran

Who survives isn't random. You did something wrong on the final mission. You have to have everyone loyal and you have to make the right selections during the mission. There is usually two right options for each task.

Well yeah, two people on my team weren't loyal and one was missing. I figured that had something to do with it, but who died seemed kinda arbitrary.

I didn't do anything wrong. Mordin was loyal, I chose specialists for all the tasks, and at the end I just chose my squad, I didn't chose anything about the people left behind. When it worked, all I did was include Mordin in my squad instead of Grunt

You have to choose specific specialists for each task or people die no matter what their loyalty rating is. These are the correct choices, if you deviant from them someone is going to die. Chose Tali or Legion to hack, Samara or Jack for the Biotic force field, Miranda or Garrus to lead the second team, and as for who leads back the survivors chose who ever I didn't pick to lead the second team.

The most annoying for me so far, is the Mechs on the lose missions, until I upgraded mech damage. 

All I say is that you should pick Garrus not Miranda.

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Spoilers I thought you had to break up the M & J catfight in a certain way? Otherwise your least fav. dies. I'm in no rush for the last achs. anyway. Reason for a fourth playthrough.