I know I’m joining in real late here but rabbiofberlin, the Steipler Ztz”l in Chayei Olam (in my humble opinion one of the GRETEST mussar seforim EVER written) explains that “Kol yisroel yesh lohem chelek l’olam haba” means IF THEY DON’T MESS IT UP!!
For Instance “Hamalbin p’nei chaveiro b’rabim AIN lo chelek l’olam haba.
Ayin sham where he explains this in depth.
By the way, as I’ve pointed out numerous times, if you take a look in Koivetz Ma’amarim from R’ Elchanan Hy”d (which you say you have) you will see clearly why the tzionim reshaim are not getting anywhere near Olam Haba.
“rabbeinu” GMAB
“prove it”
In case you haven’t noticed, about 4 people already pointed out that the Kuzari says it Mefurash.
Got that?
I think you should stop while you are not too far behind.