Don't forget Obito had the Biju helping him. You thinking that Obito didn't think Itachi could beat him is irrelevant because he stated 3 separate times that Itachi could beat him. So Obito knows already that Itachi can counter Kamui, like how Kakashi and Minato did.
Kakashi counters Kamui because he has Kamui,which Itachi obviously lacks. Minato counters Kamui because of FTG/his superior speed compared to Itachi and Obito. So do you think Old Hiruzen>onoki because Kabuto a master spy who likely knows all their abilities said so. Look at the feats and not just character's words. Itachi has no offensive counter to Kamui. When Obito took his bijuu back in the Mazo Naruto still couldn't touch him without Kakashi. Again Itachi has no feat to Obito handling 3 kage level ninja.
2.
Obito pretty much counters everything in Itachi's arsenal.
Susanoo, Yata Mirrior, Totsuka-intangibility. Itachi has no attack that will counter the 5 minute limit of intanglability especially with his stamina..
Genjutsu-breaks out of it. Sasuke broke out of Tsukuyomi with 3t Obito should have no problem considering he put a Perfect Jin under genjutsu.
Obito is also much faster. Itachi was fighting on par with KCM Naruto clone, while the main KCM Naruto couldn't even touch him.
Obito already canonically countered Amaterasu with Kamui plus it was a suprise amaterasu.
Obio can manhandle Itachi's clones like he did with Naruto's.
Once Obito touches Itachi it's over. Itachi doesn't have FTG to escape.