Mdd, it might be predictable, but it’s what the vast majority of gedolim will tell you if you ask the shailoh. You’re quoting a lot of 2nd hand information; i advise you to ask gedolei yisroel if the halacha lemaysoh follows the chazon ish. I highly doubt that rav elyashiv told someone to celebrate a mumars death, but if he did, what was the case? Was he perhaps a former rosh kolel who went off the derech and tried to convince others to do so?(it’s happened…) Or maybe a guy in a black hat who believes in transing young people, like a former kiruv rabbi turned walking abomination, who lures young people in BMG?
There are people who i think even the chazon ish would agree are in that category. I’d celebrate if either of the above g’pagered.
I don’t remember tosfos on the gemara; will check bl”n.