Yea it can be argued that at different points throughout the film, different people seem to be acting as Underboss and Consigliere, just general advisors and "right hand men" to Michael, namely Neri, Connie, Vincent, BJ Harrison, Zasa, and maybe even the Abbanando kid at points.

As previously mentioned, Michael seems to have thrown off the traditional family structure as he became more of legitimate businessman and left the more criminal aspects of his business to underlings.

If I had to give concrete answers, and also assume that Michael was still abiding by the family structure, I would guess Harrison was his Consigliere, Neri was his Underboss, and although Connie held no official position seeing as she was a woman, she was also very influential and always had Michael's ear.