It doesn't mean she won't die at some point(s). She can die in the harem/dark route and then be resetted back to life. She can die as a part of some other character's route. For example maybe the Yumi route will have Chinami dying from her illness and Chika kill herself as a result.
Thinking about it, i'm honestly having a hard time picturing a Yumi route where Chika lives. How would that work, really ? If Sensei and Chika are still "dating" Yumi won't interfere. If they break up Yumi will hate Sensei for breaking Chika's heart. But if Chika dies (and it's not Sensei's fault) then Yumi ending up with Sensei suddenly makes a lot of sense. I suppose it's also possible for Yumi and Chika to have a massive falling out but i find that less likely.
Nobody has actually died inside the bubble. Tsuneyo's father is the closest we've gotten, and honestly, I think he's a foreshadowing of Chinami's fate -- hovering right at the point of death indefinitely, but never crossing over.
Makoto has tried to kill herself multiple times and it never works. We literally see it play out in Bluejay.
Death is not allowed in Kumon-Mi. No matter how much the living may wish for it.
And if someone managed to die despite all that -- they'd be fine as of the next reset. If the resets can un-impregnate Ayane, they can also un-murder whoever got in Ami's way.
As for Yumi -- she may or may not know about how Chika feels about Sensei right now, and that's based entirely on your decisions. If you got the extended locker room scene with Yumi and Nodoka, she doesn't know yet, and some form of relationship with Yumi is likely. And if she's already IN the relationship and emotionally invested when she finds out, it'll be too late. We'll get some conflict, possibly a resolution. Maybe a throuple, maybe two equally broken hearts.
Honestly, a Yumi/Chika throuple is the optimal outcome.