There are basically five schools of thought when approaching the issue of birkot keriat shema post-plag and pre-nightfall (assuming it is not tarti desatri which has, ummm, fewer) because it seems odd to say brachot on shema when you are seemingly not fulfilling the mitzva. They are:
Rav Hai Gaon (quoted in Tur OC 235 and Rosh Brachot 1:1 and more): Daven the Shmone Esrei and say keriat shema with its brachot at night (ie skip semichat geulah letefillah).
Rabbeinu Tam (Brachot 2a): The proper time for keriat shema is after plag, so that's why one can say its brachot then.
Rabbeinu Yonah (Brachot 1a in Rif): Like Rav Hai Gaon, except that after sunset and before nightfall, he thinks that is considered night enough that the brachot are not lies.
Possibly Rashi (Brachot 2a): The brachot are part of the tefillah and can be said at any time the tefillah can. (I say possibly because Rashi doesn't say this, but he doesn't explain any other way explicitly and this opinion is how I always heard it understood. If I find someone who says this explicitly I will edit it in. IIRC Rav Soloveitchik is cited in Nefesh HaRav as holding of this kind of opinion from sunset onward, not plag.)
Gra (Maaseh Rav 65): Never ever say any part of maariv before nightfall no matter what.
Leaving aside the Gra because no one before or after him (that I can find) ruled like him...
And leaving aside Rav Hai Gaon because it doesn't meet your criteria despite its being the earliest discussion on the matter, its being cited approvingly by the Tur and Beit Yosef, its being reported by the Chofetz Chaim's (as quoted here) son that this was the Chofetz Chaim's personal practice, and its basically being the final psak of Rav Moshe Feinstein (Igrot Moshe OC 2:60) and Rav Hershel Shachter (as quoted here)...
We are left with three options:
For Rabbeinu Yonah, if you want to extend his definition of sunset back to plag, you would still be hard pressed to be able to say the brachot before plag as that would certainly be lying.
For Possible Rashi, it would seem that you wouldn't be able to say the brachot before plag as then it isn't the time for tefillah.
For Rabbeinu Tam, there is a possiblitly that one could say the first two brachot before plag so long as keriat shema is said after plag, which according to him (and against an explicit mishna Brachot 1:1) is its appropriate time, because then the brachot are still appropriately attached to the mitzva. However, the 'lying' issue may still bug him. We don't know for sure.
Note that all of the above is true even for someone who did daven mincha before plag!