I just saw this thread and permit me to say that there is a major misconception in the whole thread. Whether you “daven” mincha before or after the “shkiah” has nothing to do with Rabbeinu tam’s shittah or the geonim’s shittah.
It has to with whether you can “daven” mincha till night or not. PLEASE check shulchan aruch “siman 233-se’if 1” and the remo.
Nowhere is there a mention of Rabbeinu Tam or the geonim-which really has to do when is nightfall.