I am baffled here, how was IM about to die? He took some serious hits from Cull when his eye was off the ball and as shown with his fight with Thanos he could take incredible damage (broken portion of the moon hitting him at high velocity with no visible damage) so how on earth would that strike from Cull even dent him? If Spidey could catch that mid swing hit then it would not even have scraped the paint on IM and considering he was blasted around half a mile through buildings with no damage, again why would that last hit have hurt him at all? Also at the start of the confrontation IM staggered Cull with a left hook (and not one of his amped punches even) and then blasted him into the distance with his nano repulsor attack. Pretty obvious nano IW IM was scaled as a bit too much for Cull if it came to a long slugfest. Also a powerstone blast from Thanos couldn't get through IM's shield then there is nothing Cull really could do to IM. Spidey is very strong and fast but is an absolute league below IM.
Some have commented that Thanos was playing with IM but his pride and self belief would have wanted to beat IM with his own abilites but he had to resort to the gauntlet and specifically the powerstone a few times to beat IM.
I agree with most of what you said but I don't think it's true for Cull to not be any type of threat to Iron Man. He overpowered nanotech Iron Man's bullrush with the first attack he landed with his weapon. His weapon is also strong to completely deflect repulsors. I also thought it was impressive he could no-sell most of Iron Man's projectiles in EG and IW. I don't think he's as strong as Iron Man(Tony probably would've did better using the fighting style he used against Thor in Avengers 1 there.). I would rank him somewhere between Hulk and Iron Man though.
Also Cull didn't to try crush Spider-Man or continue his strike when he got his attack "caught". He stopped applying any pressure when he saw Spider-Man there and instead opted to toss him aside.