Are any important NPCs doomed to die, or immune from death?

WARNING: Heavy usage of TVTropes links follows:

A compelling element in the Mass Effect series is that many significant NPCs have the potential to suffer Permadeath in battle, or at other key points in the game. Many of these cases are made especially poignant for the player because the fallen NPCs could otherwise have re-appeared in the next game, and/or they died because of decisions made by the player.

In my first play-through of the series, I've had my share of player-influenced NPC deaths:

- Ashley Williams
- Kasumi Goto
- Zaeed Massani
- Mordin Solus

While all of these characters are important contributors to the story in their own way, I was a bit grateful that they were not significant as some others. One was obviously a non-optional loss (someone had to die in that spot). The rest did not survive the first episode in which they appeared, and/or were relatively minor NPCs anyway. Still, the game does fairly well to establish for the player that Anyone Can Die.

However, I'm coming along far enough into Mass Effect 3 that I'm starting to consider my second whole-series play-through. I'm thinking of having the second run be just a little bit more planned out than the first. To that end, I'd be interested to know if there are any important NPCs (i.e.: those who are available as squad members in any Mass Effect game) whose deaths are beyond the player's control?

So, is there anyone who has immunity via Plot Armor or is doomed to Death By Plot? Or are all major NPCs, by the end of the Trilogy, equally likely to have survived or been killed based on some player-controllable factors?

(NOTE: I haven't gotten to the end just yet, so I may still be missing out on some Disney Deaths, or the possibility that Everyone Dies. Annotate spoilers appropriately, please.)

In short: Which major characters' deaths or survival (if any) are not controllable or influenced by the player's decisions?

According to your comment, you're only interested in recurring or important characters. (I am not counting the post-gameplay ending of ME3 here, which has very obvious consequences based on choices you make in the ending sequences - you will know what you're getting into.)

To summarize without spoilers, when you ask "Are all major NPCs, by the end of the Trilogy, equally likely to have survived or been killed based on some player-controllable factors?" the answer is "Absolutely not".

In light of that, let's start with squad mates, since they tend to be more non-obvious and have more ways in which they can die...

In Mass Effect 1:

You can shoot Wrex at the beginning of the Virmire mission, but this is entirely optional.

Either Ashley or Kaiden must be left behind to die on Virmire.

All other squadmates will live through the game - note that it's possible to skip recruiting a couple of them!

In Mass Effect 2:

Every single permanent squad mate can potentially die during the Suicide Mission. This is all of: Tali, Garrus, Miranda, Jacob, Jack, Grunt, Thane, Mordin, Legion, Samara, (or Morinth, if you let her kill Samara during her loyalty mission), Zaeed, and Kasumi.

Shepard can also die during the suicide mission if fewer than two squad mates live (and if he does, you can't import that data to ME3).

In Mass Effect 3 (including previously-playable squad members who aren't playable in ME3):

Any squadmate can be killed at the final mission if your readiness is too low and you take them in your squad. EDI won't die (she's in the Normandy's computers), but her physical platform will be destroyed.

Tali will commit suicide if you don't properly handle the situation with the Geth. Legion will die here in some manner or another no matter what you do.

Mordin can die on Tuchanka - if Wrex is alive at this point, it's impossible to both cure the genophage and have Mordin live.

If you sabotage the genophage cure, Wrex will figure it out and confront you, I believe he ends up dying at this point regardless of what you do.

No matter what else you do, Thane will always end up succumbing to his illness if he has made it that far. This will happen off-camera if you don't visit him in time.

Jack will be turned by Cerberus if you don't help her on Grissom Academy, and you'll have to kill her.

Miranda will die if you don't warn her about Kai Leng.

Zaeed and Grunt can die when they show up if they weren't loyal in ME2. Kasumi maybe dies, but it's hard to tell since it looks the same even if you end up finding that she lives.

Samara can die during her mission if you don't stop her from committing suicide. Morinth won't show up there, but will end up turned into a Banshee by the end of the game.

As for non-squad mates:

Notable recurring NPCs who will die in the course of the series, at some point or another, no matter what you do:

Anderson, Udina, Illusive Man, Kai Leng, Saren, Matriarch Benezia, Nyreen, the Clone

Notable recurring NPCs who may or may not die depending on your actions:

Dr. Chakwas, Kelly Chambers, Engineers Donnelly and Daniels, Barla Von, Admiral Xen, Admiral Koris, Captain Kirrahe, Eve, Conrad, Balak, Shiala, Wreav (this one hinges solely on how long Wrex lives, you won't directly influence it), Maya Brooks

Also depending on your actions, the entire Geth race and the entire Quarian Migrant Fleet (although presumably a few Quarians elsewhere would have survived).

Notable recurring NPCs who won't die, regardless:

Joker, Hackett, Aria, Aethyta, Bailey, Engineer Adams

I've cleaned this up a bit to fit into the scope of 'not controllable or influenced by';

In Mass Effect 1, one of your initial squad mates (Richard L Jenkins) will die:

As you walk around a corner within the first few moments of the first mission on Eden Prime, during a cutscene. This is to make room in your squad for Ashley Williams a few moments later.

The other uncontrollable death occurs later in the game - while you have control over who dies, you do not have the opportunity to save both of your squad mates...

On Virmire, you have to choose between killing Ashley or killing Kaidan

In Mass Effect 2, the characters whose deaths you have no control over are:

The entire crew of the Normandy SR-1, in my opinion none of these are 'major' characters although some of them did have a few lines to speak during the first game so I've included this to prevent an omission. The crew dies when a collector ship destroys the Normandy during the opening scenes of the game.

In addition, when doing Samara's loyalty mission:

Either Samara or her daughter Morinth will die during this loyalty mission.

Finally, during Mass Effect 3, the following characters will definitely die and you have no real control over it. The first assuming that he survived in Mass Effect 2;

Thane is very ill, and will die during the game in hospital on the Citadel.

During the missions on Rannoch;

Legion will sacrifice himself and merge with the Geth.

And finally, although you don't have a choice on him dying, you do have a choice on how he dies;

At the end of Mass Effect 3, the Illusive Man will die, either you'll kill him, or he will kill himself.