Damn bruh I'm on my phone and wasn't expecting a reply this big. To save time and effort I won't really quote and address all your points, I'll just make my case and address what I think matters.
Your post was set up as if he fights them all 1 vs 1 when the fight is 3 vs 1.
At the start they all bring out the boss trio to combat his techniques. This alone would overwhelm him. Everything in his arsenal is utterly useless against the boss trio except his final Susanoo.
Manda and Orochimaru survive any Amaterasu attempt by shedding their skin with no difficulty.
Tsunade and Katsuyu survive Amaterasu by Tsunade hiding in her body and when Amaterasu does hit Katsuyu, Tsunade is safe and she removes the flames by dropping off that part of her body, just like the Jubi did.
Jiraiya survives it himself with Needle Jizo covering himself then shooting the flaming hair off of him. If Gamabunta gets hit he seals the flames with his fire scroll lie he did in canon.
That's if Amaterasu even manages to hit either of them since they will all be high in the air on their summons head and it will make it harder for Itachi to get them in his LoS, especially if they are spitting large scale acid and fire attacks which would bring up tons of smoke and cloud his vision.
Any attempts to genjutsu one of the boss summons is bad for Itachi. A fairly good amount of chakra is needed to control summons that powerful and if he gets 1 under his control, 2 or even 3 is impossible for him so in the end it's a waste because Jiraiya would summon another boss toad and Orochimaru would summon Aoda. It's all a big fail for Itachi. He wastes lots of chakra controlling 1 boss only for it to be beaten by 3 other bosses not to mention of they get near it they just break the genjutsu so he's back to square one, only with less chakra than before.
Same with Amaterasu, using it gets him no where and he wastes an enormous amount of chakra for nothing.
Susanoo is all that matters in this fight. Itachi stamina is low so he can't keep it up long and they will beat him this way. Manda dodges any totsuka stab attempt by going underground then resurfacing else where. Gamabunta can leap several hundred meters in the air no difficulty and dodge. Katsuyu would be the vulnerable one but she's just distracting him with acid blasts while he tiers out. Does not matter if she gets sealed or not, that's not even her real body, but 5% of the real thing and her purpose is just that.
Blitzing them is not happening, not on top their boss summons and not when they can fake him with clones, I mean at least Orochimaru and Jiraiya can.