First time I did the mission I sent Jacob through because he volunteered in the cut scene.

I didn't make a mistake my first playthrough. Everyone survived. It wasn't until my third playthrough where someone (Thane) didn't make it bc I felt like doing things differently by having Miranda do the biotic field instead of Samara.

I got lucky on the first run I guess cause if I remember correctly, I lost Miranda's loyalty in the confrontation so I didn't choose her for anything. I went with the obvious choices like Legion in the vents and Samara for the field (she is the first one to speak up about it) so everyone survived. And then on my subsequent playthroughs of the original ME2 I would use the same exact method just to be safe >.>

None of the above. I sent Legion back to the ship with the survivors because I wasn't sure how much danger they would run into, and as a result my combat rating was lowered so Mordin being the weakest was killed during the rear guard action. According to the official stats, apparently this is the most common mistake people make. I say it's rigged, you can do everything right and still get screwed over by a hidden feature.

^I did the same thing but I sent Grunt back. And it does feel very rigged every other choice the person you send affects the job this is the only one where it affects something that seems unrelated.

I brought Grunt and Garrus to the boss fight. I lost Jack in the aftermath.