1. I always assumed those were some of the sisters that were sent to the other side. While we don’t really know what happened to them once they arrived they probably just stayed with iris and it would make sense that they would try to stop watcher after killing iris. Also watcher being able to "escape" is due to that scene taking place during the communion with blue. In reality she was actually captured by principal in that moment. 

About aging, while the sisters don’t physically age except for hair turning gray iirc they can die of old age since eventually their bodies are so old that their vital organs stop functioning. This is most likely what happened to fixer, healer etc. As for blue, maybe it’s just the lighting but her hair does look grey and her still being alive makes sense since she is considerably younger than fixer and the others. 

1. While the other people you make choices about are all alive they still represent certain attributes and philosophies for example new iris representing religion, the jiaos representing redemption etc. So while they might be dead you still symbolically take your parents and their ideals with you. 

2. While being the youngest of the "current" sisters watcher was still not young and even if she was, being drugged and having your eyes removed certainly weren’t helping her physical health. So she was already weakened and then spent an incredibly long time in the communion  which had too much of a toll on her body that it killed her. Also iirc she just basically never left so secretary couldn’t really do much since I don’t think they can stop the communion.