This is the simplest of the rescues, and allows you to "save" Drass without sending him off with the desire demon.  Instead of going hostile when the desire demon attacks, Drass will stand there befuddled.  If you speak to him after, he'll offer a brief apology.  The codex that is normally looted from his body ("Desire and Need") can be obtained from the desire demon's body instead.

Vanilla files altered:
-- cir000pt_encounters.ncs
-- cir220_desire.dlg
-- cir220_desire_amb.dlg

Niall can now be encouraged to not give up hope in his final conversation before leaving the Fade.  (My theory on this being his concept of time while in the Fade is a bit askew, and while he is weakened in the real world, he doesn't need to die.)  By default, Niall will still die.  You need to actively choose one of the two dialogue options telling him: "Don't give up hope."  You should have several opportunities to give him this prompt, so as long as you don't just tell him you're leaving, he can be saved.

Once you exit the Fade, instead of looting the Litany from his body, speak to him.  He will pass the Litany on to you and urge you to defeat Uldred.  Afterward, if you spare the mages and they join your army, Niall can be found in the first room of the first floor of the tower, not far from where you overhear the two mages discussing the Blight.  Speak to him, and he will give you a small reward (a ring) for saving him; the stats on this ring change depending on what line you used to encourage him earlier.  If you don't want the ring or dislike the stats, either don't speak to him, or toss it.

Vanilla files altered:

-- cir300_niall.dlg
-- plt_cir000pt_litany.plo
-- plt_cir300pt_fade.plo

The two plot files are codex entry changes and only needed for immersion; if you have some conflict, they can be removed without issue.

By far the most complicated of the NPC rescues.  Please be sure to read the details!

Nelaros will no longer be killed upon reaching the hallway with the guards in the Arl of Denerim's Estate.  Instead, he will be on the ground injured, but will get up to fight with you when the guards become hostile.  Afterward, he will speak to you and (optionally) join you as a party member.  Once you return to the Alienage, dependent upon your choices with Vaughan, you will be given the option to speak with him before you leave for Ostagar.

Nelaros's outcomes are dependent upon your choices when speaking with him before the wedding and upon your decision with Vaughan.  Nelaros has three states he can be in after his intro conversation in the Alienage:  Angry, Rejected, and Neutral/Positive.  If you made Nelaros angry during that introductory conversation, you have the option of telling him to get lost when you encounter him at the Estate.  (Tell him:  "You're pathetic.  Leave before you get yourself killed.") Nelaros will only leave at this point if you have previously angered him.  If his status is rejected or neutral, he will refuse to leave (though this will change his status to rejected if it wasn't already).  So if you don't want him coming along with you for the rest of the area, make sure to piss him off first.  If he leaves the estate, you won't see him again.  Presumably he's off in Highever, living his best life.

For the sake of immersion, Nelaros joins the party with a couple of injuries.  One the guards you fight will drop a sword to equip him with.  You can steal his clothing if you really want to, but as this particular clothing model doesn't have a female variant, I've restricted it to male characters only. 

If you choose to accept Vaughan's bribe, Nelaros will object.  If you go through with it, he will leave your party before the next cutscene, and will not return to the Alienage with you and Soris.  (He cut his losses and ran, and good on him.)  There will be no reference at all to Nelaros after your return to the Alienage if you accept the bribe; Soris won't mention his death and Valendrian won't ask after him.  Honestly, I wasn't all that concerned with making this option immersive.  If this is the route you want to go, you get what you get.  You also cannot implicate Nelaros to the guards upon your return.

As a bonus, I also made it so Vaughan always drops his Noble Clothing, and just because they're jerks, his friends now drops theirs, too.

I did not include Nelaros in either of the cutscenes after defeating Vaughan, as his placement in them would have felt awkward for those specific conversations.  I also didn't edit the "we'll make short work of these two" line in the Vaughan confrontation scene, as I find it reasonable to assume a scumbag rapist would dismiss the woman as a threat and simply refer to the two men instead.

When speaking to Nelaros before leaving for Ostagar, dependent upon your dialogue choices, he will offer you the Wedding Ring item that you normally would loot off his corpse.  You can then either accept it or tell him to keep it.  (You can still get the ring even if his previous status was Angry or Rejected, you just need to be nice to him during his final conversation.)

Note on the Writing and Audio:
Nelaros is very close to a blank slate when it come to characterization, so I basically had to wing it when coming up with his dialogue.  That said, I'm not sure I quite nailed it.  Additionally, his audio isn't as close a match as Niall's is, as the synth couldn't fully get his intonation.  My personal headcanon here?  He was trying to impress you at first and had a bit of an affectation. ;) But if you prefer the conversations to be unvoiced, you can remove his audio easily.

Vanilla files altered:
-- 5p_guard_captain.stg
-- alienage_return_rescued.stg
-- bec100ar_elven_alienage.ncs
-- bec100pt_soris.ncs
-- bec210ar_estate_int.ncs
-- bec210cr_palace_captain.dlb
-- bec210ip_arl_bedroom_door.ncs
-- found_vaughan.stg
-- den300_shianni.dlg

Note on the Shianni Dialogue File:  this file is also altered in Zevran Dialog Fix and will be incompatible with that mod's version of the file.  However, I did what should be a similar bugfix in my file.  That said, this file is only necessary to alter the "He died" dialogue option when returning to the Alienage, and you can always ignore that option if you prefer not to use my file.