I read through your post to waltz.
For one you assume the sand has intel on Itachi due to jiraiya giving them intel regarding akatsuki which is very vague. Sure I will not throw out the possibility that gaara potentially has knowledge on his MS techniques but it's all fan fiction as of right now. Regarding Dr.proof posts his main argument for tysukuyomi is the range length at which it can be activated on to the victim, I have already listed counters to that as seeing per say Gaara is on the surface, Itachi can simply close the gap with his shunshin or using his dusk crow genjutsu and simply shunshin blitzed his clone over to gaara and have the crow clone to activate the genjutsu, or rather simply perform his finger genjutsu which gaara has no knowledge on unless your implying gaara by accident won't look at his finger by accident, which is fan fiction at it's finest.
If gaara takes to the flies which I already went over via Itachi's speed and height at which he can jump with the combination of
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which he will then proceed to crush gaara on the spot and no his sand armor isnt saving from getting crushed via sussano when all it took was rock lee chunin exam punches to crack his sand armor. Or once again regarding gaara taking flight, Itachi can simply send up Crow clones and make them re form as Itachi and use dusk crow genjutsu.
Regarding 3rd eye, Itachi should be capable of spotting it rather easily as he a
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. With the use of explosion tags and etc and making clouds of smoke, itachi can easily cloud his location and prep for a amaterasu. Regarding gaara's sensing sand, it will be troublesome when the victim makes contact with the sand I won't deny will be a pain in the ass.