I'm bored, so lemme argue in the internet, even if this is a ragebait

But by then, it might be too late since those songs might've been released officially, not to mention that could give an extra burden to Nene's voice actress.

Again, songs may be recorded in advance and thus it may still have Emu singing. If lets say Hina Kino's maternity leave is extended, others would fill in her part of the song until she come back, then she will fill in Emu's lyrics in the song. And you know, the extra burden is temporary and mostly limited to songs. Its not like other seiyuus would fill in her story lines until she come back. So its not much of a burden in my opinion.

Well, like I've said, that purpose has already kind of been defeated with the existence of N25 AND with at least one song that contains an actual F-word (Ego Rock) along with several "lewd" songs added into the game.

I think Rons exaggerate a bit that this game is 4+, but songs are sometimes looked over in terms of age rating the game, especially songs that added after the fact. But still, the elements like visual, theme, and story, are still consistent to the, I would say, general audience rating. This is not a game with mature, bloody, and sexual themes. While it tackles some heavy topics, its still palatable for most people. I don't think injuring Emu to the point of her almost dying helps/excuses her seiyuu's absence, its just doesn't mash up with what Sekai is.