There is
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, and there is
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. The frog song has no prep time. It takes effect in an instant and paralyses the opponent in a similar manner as Kabuto's
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The frog genjutsu, Jiraya won't need to come close to Itachi. Infact it's Itachi the one who will need to stop the jutsu before the end of the song since it's a long range technique that covers a wide area, so Jiraya can do everything he can to go away meanwhile and Itachi would be the one affected if he doesn't manage to stop him in time.
And what is his counter to Yomi Numa?
Oh wow Thats my bad, Confused the two because of their name but frog song is of no factor here to be honest, first off that's assuming jiraiya will be capable of summoning Ma and Pa in time. Anyway's regarding frog song as noted Ma and Pa first form
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Itachi having a very high perspective of his surrounding should be capable of noticing this and respond with a offensive counter to them both ending in their deaths. Also Itachi being temporarily paralyzed should not him from performing Eye doujutsu techs since they require no hand movement via hand seal
Your analysis on the frog genjutsu is arguable.
Depending on the location, lets say a outside place, the song will quickly give away their position noted by
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Here is when Frog genjutsu starts
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This is the duration process until it's complete
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Taking into factor of all that time, Itachi can easily end this with a amaterasu of some sort or yaska magatama's if needed, anything that will reach them and cause a deadly blow. And regarding Yomi Numa, I see no reason why he can't use his sussano arm length and simply extend it until it grips on to a solid structure or the surface and bring his way back up possibly.