Itachi cant possibly win.....unless u hv him edo i.e. fully recovered with no disease and unlimited chakra(stated by Madar)
If shinra tensai can destroy the leaf,what makes u th8nk it cannot destroy susanoo
If u put case of yata mirror it only defends in front rest sides r open...
Yasaka magatama cannot alone destroy chibaku tensai...
The summons r overwhelming especially the dog but Itachi can deal with them
If rinnegan is the strongest dojutsu,then genjutsu wont affect it (most probably)...
Nagato is a sensor and was able to tell when Itachi was going to use amaterasu in mang
Moreover later on nagato removed amaterasu by using shinra tensai...
Preta path can absorb almost all of Itachis attacks....
Asura path can demolish a lot of stuff and by my guess even the initial stages of susanoo..
The only problem is nagatos mobility....but that can be solved,also nagato posseses all element due to rinnegan(refer to wiki)
So basically,
Edo Itachi>Nagato
Nagato>Itachi
Edo Nagato>>Itachi
Edo Nagato>Edo Itachi