This has been covered. That's not really what he says. He is surprised a single guy could beat Hanzo, but that's after we're explicitly told about all Hanzo's security precautions and such. He could equally be surprised someone he's never heard of could do all that. And what is Jiraiya's reaction after? He doesn't say "gee, if this guy could single handedly get past all Hanzo's security and still beat Hanzo, he must be stronger than me". Instead he says "yeh, this guy could be strong, I'll take the key out as a precaution, but I think I'll be fine to go fight him... shouldn't take too long". So clearly Jiraiya is backing himself to beat someone who was apparently much stronger than Hanzo.
Mhh, you do realize that you don't need to be stronger than someone to beat him? Jiraya originally infiltrated to obtain information on Pain. What he found out, led to the flashback, where the elder said, he one day would need to make a very important choice. Jiraya thought that day had come, and he knew he had to face his former student.
Also you are terribly wrong, thinking Jiraya went there to fight Pain. It was Pain that engaged Jiraya, rather it was Konan. Jiraya had disguised himself as the guy he interrogated.
When Jiraya learned Pains true strength, he ran away for a moment, then they tried a sick OP genjutsu, and even later, when he could still escape, he made his "choice".
Truly IMO Hanzou is kage material. He led a country for long, and showed explicit leader material through good negotiations and such.
Well, we don't see them, and even in the anime (which suggests they had them during the war), the summons are depicted as substantially smaller than they would grow to be. It may just be that by the time Hanzo rocked up they were too exhausted to use them. I'm not going to just grant that the Sannin had them 25-35 years ago, just because it suits your argument. Tsunade didn't have her seal, and we know she hadn't developed regen yet (as you admit) because Oro didn't know about it... which says nothing of her even better Regen tech, which she didn't use until the Madara fight.
Yeah, they were smaller, but they most likely still had them. We don't know about their strength at the time. The only early picture we saw of Gamabunta, he is still quite large. And this is, when Jiraya is like 12-15 years old. I simply wanted to point out that it's more likely that they actually used all summons in that particular fight.
There is a world of difference between knowing how to use Sage energy, and being able to go into Sage mode. There is no evidence Jiraiya went into it, and if he did why would he have stopped using it during the fight (given it not only makes him vastly stronger, but doesn't run out of energy)? It's pretty unlikely he used it, or was even able to use it yet.
Hmm, Jiraya was never able to use it. He needed to summon both elder toads. If they admitted defeat, he would unsummon them. That would be natural of course. I also don't think he used them though. It seems his relationship with them is rather personal, but still if it was to protect his comrades, I have no doubt that he would summon them. Again, no evidence.
This has all been gone over in depth. There's little reason to give Hanzo much cred, and lots of reason not to (the latter evidence being more persuasive, and less hyperbolic). Repeating arguments that were long ago debunked is not constructive, and you should stop it.
Well, the way of shooting Hanzou down, is not arguable IMO. Basing it one fight, where he was in edo tensei state, is not very provable. It's not like Sasori or Deidara used their full arsenal of jutsu either, and they even wanted to fight. Hanzou was in an emotional state, realizing all his errors, as he saw the united shinobi alliance, and the person he once defeated.
I don't know, why Kishi chose Hanzou to give Mifune (and the Samurai) some credit. But
most people agree that it was just a plotted fight, which allowed Mifune to win. Besides Hanzou didn't use any ninjutsu in that fight. I think Mifune did something in the start to prevent it, so tbh, you can't really use it for anything at all. Kishi had to give the Samurai some shine, and whether it had been Mifune or perhaps one of the 3 kages, does not matter. I don't see that fight as much valid evidence. Hanzou has been given enough credit throughout the manga, but we will never reach an agreement on that, I bet.