Not even jumping will get Itachi in range quick enough to stab the core. By the time itachi is in range the core is covered by a crap ton of rocks, Hence why Itachi asked Bee/Naruto to use their strongest attacks because they needed to use destructive force to break through the rocks, A mere stab from a sword isn't going to break through that thick layer of rocks.
Even if itachi does deal with chibaku tensei (he can't), He can't deal with all of Nagato's summons and nagato himself, He simply doesn't have the chakra. Nagato can pretty much spam what ever the fuck he wants.
Also Itachi @ 30% chakra stated he couldn't use Ms techs, Meaning Ms techs take up massive levels of his chakra, he cannot spam them, He can use them only a handful of times before his chakra is gone.
Nagato's sensor abilities will sense any attack Itachi makes and sense itachi's location, plus it easily allows him to avoid eye contact.
Preta path to absorb susanoo/totska, Both are just manifestations of itachi's chakra (basically things given physical forms using chakra)