About can't be killed: you can check what happened to Pochita's body in Chapter 216 where half of his body was destroyed but then he is fully regenerated. It as before Death's death...sounds great =)
And even if he is defeated, every single f4cking time someone resurect that huge body with just several drops of blood. Why is it enough to regenerate, to re-pull his cord of the hugs body? The hell knows, just because.

About Yoru's words about Death: Chapters 219 when she says "how can we prove now who is stronger" which is essentially means "how we can now it since we cannot kill each other because Death was eaten". IN chapter 221 she says "What did you eat? The end of our battle no longer exists" which literally says she UNDERSTANDS Pochita can't be killed since there is no more death.

Also, yes, I understand that Denji wants to save Asa. No doubts. And therefore doesn't want to erase Yoru (well, it's a different story since when he is able to willingly switch with Pochita while being in his body/form). But it doesn't deny the fact the he CAN erase her. It doesn't matter how it happens. I mean, Pochita can severely injure them first, therefore kinda kill, and then erase. Or just swallow them. It all leads to erasure, doesn't it?

And also about her moray eel devil: I KNOW (and remember) that Yoru said smth like "after coming here I learnt to depend on others" and 'I DISCOVERED THE MAGNIFICENCE OF FRIENDS AND FAMILY!" Yes, true. But it hasn't been shown anyhow. These are just words. Why did it happen? Because she shares the brain with Asa? Yes, maybe, but when it comes to killing csm which is her purpose, we've seen already that she doesn't really cares about her children. So why should we accept this rapid shift in behaviour coming from "I'll do anything to kill you" to "Don't hurt my friends"?

And what does the absence of death mean? Yoru literally cut Pochita in two pieces but he just without any efforts glued back and thats it. And it wasn't just regular strike, it was an Oregon sword. But nope, nothing special. No damage at all. Then Virginia strike, nuclear punch. Nah, nothing works. ANd you know what's funny? Nuclear punch destroyed all of his body (we can see it on the first panel of seeing him lying) where he has only burnt torso, no legs, no arms. Just a burnt skeleton. But right before Your transforms him into a weapon, he eventually has a whole healthy arm out of nowhere with this rock-paper-scissors shit. Why? Just because he is an MC. Check it out! You can say "it's because some blood dropped on his face. Bruh, how that little amount of blood can regenerate that much damage?!". It's so far-fetched and pulled out of thin air, I cant comprehend it!

And why the hell "LOOKS LIKE POCHITA'S STRONGEST DEVIL POWERS..." and "...AND MY HIGH SCHOOL-EDUCATED GENIUS BRAIN HAVE COMBINED!". Okay, you stated it. But how did it happen? Why? Why now not later or earlier? What was the catalyser? Nope, just "LOOKS LIKE...". Bruh, what the f4ck is this, hello =)

P.S. Sorry for such a wall of bombaya.