With Fanucci out of the way Genco would have been able to pursue and marry his girlfriend, and he would have considered it one of many "miracles" Vito performed for him.

I'll go along with the theory that Genco got tougher as he got older, and I would add that once Vito had Tessio and Clemenza working for him, and as the family started to grow, Vito turned to Genco who had, in effect, been his like a brother. It is clear Genco's father took Vito in, gave him a living and only cut him loose when Fanucci forced him to put his nephew to work.
It makes sense that the one person Vito could trust with everything was Genco who became consigliere.

Vito's trust and his soft spot for Tom Hagen may well have been derived from his own experience, because Tom, like Vito was a virtual orphan who was taken in and raised by a family that was not his own.