Granted, it says "Spoilers" in the thread title anyway but I thought I'd reiterate that there are some end spoilers here, regarding who you send to do what in the final mission. Some people thought it was random or a glitch (as far as who lives and who dies) but a dev was nice enough to spell it out for everyone.
Mordin's death is not a glitch or bug. I believe that when people are surprised with Mordin's death, it's during the "Hold the Line" time where you leave crew behind as you and two squad mates move onward. Tech had their moment in the sun with the tunnel infiltration. A biotic is key for the protective bubble. The Hold the Line sequence is time for your soldier types to do their best.
Basically you want your best DEFENDERS to hold the line. Don't think about offensive solo strikers. Think about the guys that can dig down and hold a position with moxie. You want them to hold the line.
Under the hood, each character has a "hole the line" score, which gets a bonus if the character is loyal. Characters like Mordin, Jack and Tali are squishy and not exoeruenced with bunkering down and holding out for an extended period of time. Characters like Grunt and Zaeed are tanks who thrive on this type of work. The score is tallied for all the team members that are holding the line, and the number of survivors are calculated. If people are to die, the non-loyals squishies go first through the list to the loyal tanks. Alas, I think Mordin is at the head of the death list. Some characters can't survive without help even if they are loyal. Others can survive even if not loyal.
Here are some examples for the group holding the line:
Loyal Mordin by himself: He can't hold the line by himself. He dies.
Non-loyal Grunt by himself: Grunt lives. He hunkers down and gets the job done, and doesn't have to worry about helping any one else.
Loyal Mordin and non-Loyal Grunt: Grunt dies, but Mordin lives. Grunt is able to hold the line but goofs up helping Mordin... he was just too aggressive without his right of passage.
Loyal Mordin and Loyal Grunt: They both live.
Note it is possible to get non-loyal henchmen to survive through the end game with the proper group holding the line.
So what is a good strategy? Send back a loyal squishy with the crew, like Mordin or Tali. Leave your defensive bad asses to hold the line (hmmm, Garrus defended a base all by himself to an extended period of time....) and take your loyal offensive favorites with you to the end.
As for saving Kelly and the crew, you just need to make sure you go through the Omega-4 relay right after the crew is taken, and make sure you send a loyal squad mate back as an escort."