CS, I recently read Tommen's wedding as well. I noted Cersei's observation...
Kevan and Garlan have a number of commonalities. Both are younger sons of mighty houses. Both are loyal to their houses. Neither will inherit their father's lands and titles, but that has not stopped either from making good lives for themselves. Both have proven themselves as battle commanders and counselors.
Yet, both seem to have a great love of country and a deep desire to see it healthy and whole. Neither are so blinded by house fidelity that it prevents them from serving the kingdom. Both congratulated Tyrion on his foresight and bravery... in fact, they are the only two to do so.
Mayhaps Kevan sees in Garlan a man of character, a man of patience, a man of justice... i.e. a younger version of himself. Mayhaps Kevan sees a potential ally.
Yet, I don't think that either would plot openly against Cersei while she is watching. I think they were discussing the war. They probably talked of the Lannister-Tyrell victory over Stannis and the coming fights at Storm's End, Riverrun, and against the Riverlands' bandits.
If Olenna, Mace, and Loras were to die, then I think a Kevan, Daven, Jaime, Willas, Garlan, Margaery, and Tommen alliance could be a formidable front that would concern itself with the actual welfare of the realm. None of the members are great players, yet united they might prove strong enough to preserve an Andalish kingdom that could resist Littlefinger, Stannis, Euron, Arianne, and Dany.
