Regarding Minako's operation, what do you think really happened there?
At face value she lied to everyone about her operation date (saying it was a check-up before the official operation) so no one would worry on that day. Then quietly left goodbye cards while everyone was out "in case" the operation failed.
Case closed right?
But there is something very distressing about her absolute certainty about her death. Would she not have been better to leave some letters with Artemis to distribute in the future "just in case"?
I cannot help but think Minako only told everyone there was an operation with a slim chance of success to give them a light of hope and that the reality was there was no operation and essentially she just went to the hospital to have the opportunity to die peacefully. Given how sad this is it would make sense for PGSM to not have explicitly covered this and left the viewer to make up their own mind.
It's not revealed in the show, but X-ray films of the brain were shown in a hospital scene of her, & she also appeared to have headache in other scenes, hence easily leading one to guess that it was brain tumor/cancer.