Nothing in the interactions Vito has with Genco in GFII indicate that Genco is some sort of mastermind, or even a savvy, street-wise operator. We see him cower before Don Fanucci. We see him open and close a door for Signor Roberto.
These are scenes before Vito became a mafia boss. Remember, Genco and Vito started with the grocery store which Genco's father had owned.
Michael tell's Kay that a consigliere is a very important person. He must have observed this from Genco. When Sonny joins the "family business", it's Genco who teaches him what to do (understanding that Vito wants Sonny to be the next Don.)
When Sonny's shot, Tom faces his own weaknesses:
He had been fooled, faked out, by the Five Families and their seeming timidity. They had remained quiet, laying their terrible ambush. They had planned and waited, holding their bloody hands no matter what provocation they had been given. They had waited to land one terrible blow. And they had. Old Genco Abbandando would never have fallen for it, he would have smelled a rat, he would have smoked them out, tripled his precautions. "
