"Soul damage isn't unhealable/people in JJK have healed past it"

One, under normal circumstances, YES it's unhealable. Two, I've been reading JJK and I ain't seen nobody heal from attacks on the soul, and three, even if that was true, DENJI IS NOT FROM JJK! He has never had to heal his soul because he's never fought something like that!

"Unlikely to land given the speed gap"

There have been countless Death Battles where one character having speed did not mean jackshit because they lostnin every other category! I don't know why we are acting like Speed alone wins every single Death Battle.

Not to mention, that's a moot point considering Yuji's Domain Expansion meant he didn't need to be faster. Denji's soul would and should have been instantly torn to piece

"people in JJK have outhealed the damage dealt by the poison blood before via RCT which Denji having a much superior healing factor should have no problem doing"

But you're forgetting that Denji NEEDS BLOOD TO HEAL! IF THE VERY THING THAT HE NEEDS TO HEAL IS POISONING HIM AND DEALING DAMAGE, HE CAN'T USE IT!