Is there any official statement available to dwell on the relationship between Clemenza and Frankie and what on earth happened after Micheal left NYC?

The only "official statement" concerning the succession was Michael's to Frankie in the boathouse: "Clemenza promist the Rosato brothers three territories in the Bronx after he died... You didn't give them to him." The only thing we can be sure of is that this tells us for sure is that Frankie was Clemenza's successor--probably (but not necessarily certainly) his immediate successor. We know nothing for sure about what Frankie did under Clemenza, or how he came to be Clemenza's successor.

We also can be sure that either Frankie or the Rosato Brothers are mediums who can communicate with the dead (i.e., "Clemenza promised ... after he died."

Did Frankie also inherit Tessio's territory in Brooklyn?

Good question! It points up a major disconnect in the novel and in the transition to GFII:
The novel says that, within 24 hours of The Great Massacre of 1955, Neri was sent to take charge of Tessio's regime. But, within a year of The Great Massacre, Neri had become head of security for all Corleone hotels in Nevada; "Clemenza had his own Family. Rocco Lampone was Corleone caporegime." This confuses me: If Clemenza finally did get his own family, what did it consist of? If Rocco was Corleone caporegime, where was he operating--New York or Nevada? If New York, it implies that Michael still had a criminal enterprise or territory that was in some way different from Clemenza's territory. If Nevada, it implies that Michael had illegitimate interests there in addition to his legal casinos and brothel.

Finally, we can conclude from the immediate aftermath of The Great Massacre that Michael valued Neri more than Rocco because he put Neri, not Rocco, in charge of Tessio's regime; and took Neri, not Rocco, with him to Nevada. This wouldn't be surprising, given that Michael recruited Neri and Clemenza recruited Rocco while Michael was still a "civilian." But I infer from the year-later scenario that Neri got nothing better than a job as chief rent-a-cop in Nevada, while Rocco seemingly became a real pezzanovante.

The film neatly ties up all the loose ends: both guys are with Michael in Nevada, and Frankie is definitely not independent of Michael.

Although I think that Frankie may have thought otherwise when he said to Michael, at the meeting in Nevada, that he was trying to tell him how to run HIS family. But Michael immedeatly puts him in his place with his response that HIS family is still called Corleone.