tobi/obito can't kamui anyways without first materializing to grab itachi anyways.lol
second, the sword is spiritual. think about it this way, take a box and put up four walls, perpendicular crossing each other creating four seperate spaces. these spaces are known as dimensions and there are theories about multiple dimensions. the spirits or ethereal objects are outside of that, its like you(the person/spirit)looking into the box and seeing all four dimensions, thats how it works.(in theory anyways)
so conclusively we cannot draw any concrete information, but based on knowing how ethereal/spiritual things work, it can theoretically be said that the sword of totsuka can pierce tobi regardless of the dimension he's currently inhabiting. ethereal/spirits inhabit all planes.
this also means that the sword may or may not be visible. if i may give an example( its possible it could also only be visible or viewable if your being pierced much like Nagato's king of hell, konohamaru couldnt see it, yet when his life force was being yanked out, he saw it. also, i just checked, the wiki didnt state it being ethereal or not, but it did say that only sage mode naruto and itachi could see it without being in its grasps)
and tobi wouldnt be able to kamui the sword of totsuka regardless, he cant grab a spiritual item, it has no physical mass to absorb.
*sorry this is so long, i had so much to say lol*
orochimaru was not scared of itachi, but knew you couldnt defeat him. Danzo was not scared but did not show confidence in defeating him either.
tobi/obito did state" If i hadn't kept a few secrets from him myself, I'd be dead" (opinion)possibly meaning he knew itachi's capabilities and quite possibly learned a few more after he watched the recorded fight with sasuke.
i believe that with full knowledge on each other, before itachi's death(we can't include edo itachi or tobi's progression after itachi's death) if they would have fought, itachi would win. BUT high difficulty.~opinion