I don't think it's any of these reasons, I too Constanzia have pondered over this section of the book for many hours, but I too, as Tom did put on my Sicilian cap, and with some helpful objectives from my Don (my father) I have came up with a different perspective, it is, of course, purely of my opinion. Tom may not have been the best of consigleri's druring the war but he was never the less good. Michael did love him as a brother, very much so, he was in awe of him, but he had to seek his revenge on Carlo, he did not blame his brotehr Tom, he blamed Carlo, pure and simple. He had to get Carlo close enough to be certain, and to get him close without him smelling a rat he used the excuse that Carlo came from Nevada and therefore he would know the place and would be best as Mike's right hand man,. But think about it, Mike's sicilian, and both Tom and he are very clever people, why ditch Tom for Carlo? He needed to lure Carlo into a false sense of security, he couldn't afford for Carlo to fuck up his plans in this very tricky move. So keeping Tom in the dark and telling him infront of Carlo and Neri made his responses real, raw and felt, Carlo would then be at ease to know he was not being taken for a ride by Michael.
Sorry it was such a long answer but I just thought I should share my thoughts. Good question though!