I have put a link to a good guide to help you survive through the suicide mission, although basically it just depends upon you having your team members loyal.
Yes, it does, if you don't complete Legion's loyalty mission on Mass Effect 2 he won't survive The Suicide Mission, therefore he won't be in Mass Effect 3 and he is replaced by a AI of Legion.
You have to go directly to the Reaper Base and the suicide mission right after the crew gets captured. if you do any other mission or go to any other planets before the suicide mission, members of your crew will die.
You first run into Tali at Freedom's Progress,but at the end of the mission she is not recruitable as she has her own mission to take care of. However, later on you will be able to help her with her mission,which will lead to her becoming recruitable for your suicide squad.
All joinable NPCs have a loyalty mission, and if you don't complete that mission, there is a chance they may die while on the 'suicide mission' at the end of the game. If you don't do Miranda's, then she may indeed die depending on your other choices during the suicide mission, especially if you pick her for a specific task.
Assuming the non-loyal member survives the suicide mission, yes, you are able to do their loyalty missions afterwards. However if you already failed to get their loyalty on their mission, there is no second chance to get it, even after you complete the main storyline.
No, you do not need to upgrade the medical bay to have a zero casualty last mission in Mass Effect 2. The med bay will just get rid of facial scaring.
If you don't get Jack's loyalty she will die on the final mission.
legion
If you mean the "800 blocks" on the Citadel, then yes, you will go there as part of a mission.
No. He (can, if he survives a certain mission) be in your party, but you will not be able to play as him.
If he died in Mass Effect 1 then he is not in Mass Effect 2. If he survived Mass Effect 1 he shows up in Mass Effect 2 but he can not be killed.
Assaulting the Collectors' home base after passing through the Omega 4 Relay.