(I could be wrong, I have no sources to back me up)

• Roger est très vieux.

• Roger Tortue est très vieux.

  This is a weird case. For me, that would be kind of a surname, so the agreement would be masculine.

• Roger la tortue est très vieux.

  Same as the last one.

• Roger est une tortue très vielle. (Il/Elle) [both work, depending on the desired focus] aime se promener.

  This is exactly like in English saying "it" or "he" for an animal.

• Roger aime se promener. Cependant, cette tortue est très vielle, donc elle [since we spoke of "la tortue", it needs to be feminine] marche très lentement.

• Roger, comme toutes les tortues, est vert.

  Roger est vert.

• Toutes les tortues sont vertes, donc Roger est vert aussi.

General rule is that the adjectives get their gender from their noun. Choosing one or the other moves the focus toward "la tortue" or "Roger".

I would agree with Sifu’s answers, although his explanation for «Roger la tortue est très vieux», claiming “la tortue” is a sort of surname, is a little unsatisfying: “la tortue” is a qualifier or a second predicate for Roger, who remains the subject of the verb être, thus the predicate “vieux” is masculine; conversely in «la tortue Roger est très vielle/la tortue, Roger, est très vielle», “Roger” is a qualifier or a second predicate for “la tortue”, thus the predicate “vielle” is feminine.