Fireball is not "random", so your healers shouldn't be that pressed to cap everyone off. Pre-succor is very easy to do, so if your SCH isn't doing it s/he needs to step it up.

Pre: Succor, soil is pretty optional
Post: Succor, Medica2, Regen on MT

Your healers should learn the correct timing for their spells so that the cast finishes as Fireball visibly hits, so the healing is almost instantaneous and Fireball won't interrupt their casting. Can instead use Surecast or Swiftcast as previously recommended, but learning the timing for pre-healing helps your healers improve in general.

Our group puts the tank in every conflag and makes sure phase 1 -> 2 timing is synced so that Death Sentence would go off while conflag is up, so Twintania just never does it in that phase.

You may also want to just do 4-stack conflags if people are having issues surviving 3-stack. If your DPS isn't good enough to kill a 4-stack you're going to have issues later anyways. You can do: SCH+fairy+(2 others) on one side, (4 others) on other side of Twintania. Put Garuda somewhere out of the way. Or you can do a variation of what you've been doing: OT+Garuda+fairy+(fireball'd).

Other than meeting the DPS checks, if anyone dies for any reason in this fight outside of Twisters or Divebombs it's usually a healer problem.