Seems like it was axed, the suddenness of developments int the last few chapters don't seem planned. His gun seems to abruptly lose 20 lives that get handwaved away and everything is made way more urgent. Seems pretty similar to axed mangas that do timeskips and other stuff to wrap up the plot quickly.
The missing 20 aren’t handwaved away, he used them up when he returned to before Shiina’s coma. In chapter 12 we even see the 001 in shadow at the top of page 13.
Honestly I was more confused by the whole “one final suicide” angle but rereading cleared it up. He’s anxious without the use of the gun, worried about her body merging with it, and I think is trying to punish himself for killing her during the coma incident. She also feels guilt over having broken him this much and wants to feel some pain too
I would kill myself too, because how on earth do you explain to someone why your heart doesn’t beat and you have a magical heart-shaped stone in a gaping hole in your chest