Harrigan was kneeling after several strikes he had (and a net that should have killed him). Not to mention the condition the hunter was in. He sustained serious damage, and that could cloud his logic. The two cases aren't on the same level, not near.
Why would celtic assume it? Maybe because all the gear he used was demolished by the acid, and it was only a matter of seconds. And the net was fuming. Even if he assumed it's more resistant, he should have made his move asap he could. But he didn't. It's not simple miscalculation (based on overconfidance) like what the hunter in p2 did.
QuoteNo it doesn't. In the two Predator movies, the main character has script immunity.
As an argument I don't see its meaning. It can be said for the predator too. The script is written in the way because they want it to be. They wanted to show, the predator can be defeated that way too.