- the genjutsu will only be broken if someone notices it in time
- They fuse physically. It was never said they fuse chakra.
- Sasuke can shoot an arrow before or after Gama shoot his water jutsu.
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continued moving towards Sasuke unheeded in the dimension he created. Thus, his genjutsu pales in comparison to Itachi's, which Orochimaru needed time (which he lacked) to form a seal and break the illusion.
What allowed Sasuke to control/corrode that dimension was not genjutsu, but rather his will. It's rather ambiguous wording, but the most logical explanations are:
1) An alternate dimension such as that one requires some mental facility to maintain and Sasuke's was greater. After all, I'll admit that he's a genius and at the time his determination was greater. So in this case, will would actually refer to willpower.
2) Will refers instead to chakra. In this case, Sasuke used his chakra to overpower Orochimaru's and thus the jutsu. We've seen in other fights (against the Raikage and Danzou) how corrosive / cold Sasuke's chakra could become and he demonstrated the ability to power through Itachi's genjutsu with chakra (though in that case Sasuke had to use the cursed seal).
And I'll say it again, for all the people saying genjutsu is a strong point for Sasuke, Jiraiya is a clever fighter and knows his weaknesses. He demonstrated that he is prepared to counter genjutsu when he trained Naruto. As for the Sasuke, his genjutsu only gave him a clear advantage against C and Danzo, and in the latter, how it used was more important than it's strength. Jiraiya was never a genjutsu type, so he isn't likely to underestimate that ability like Danzou. But Sasuke's genjutsu never gave him a clear advantage against Bee and Orochimaru since they recovered quickly from it.
Once again, defending against Genjutsu is a different thing to using Genjutsu. Jiraiya was confident enough in his own ability to counter Genjutsu as he took it upon himself to teach Naruto, who he knew would be facing the likes of the Mangekyou Sharingan in Itachi and Sasuke with his normal Sharingan.
Jiraiya, the student of the great Sarutobi Hiruzen, would most definitely know how to defend against high level Genjutsu and his large chakra pool makes the counter perfect for him as it is for Naruto. It's safe to assume he grew up knowing full well the capabilities of the Sharingan. People underestimate Jiraya quite a lot. He has A LOT of versatility, experience and fire power at his disposal.
Quite frankly I believe all of the Sannin are underestimated, but oh well. That's just my thoughts.