The definition of “mekabel” is these examples is that the mitzvah was done. Using the term “mekabel” by sefira you were mekabel ba’al korcha the mitzvah of counting and by being yotzeh the counting you can no longer re-do the mitzva, it was done.
If saying “good shabbos” on its own is kiddush as R’ Akiva Eiger suggests and therefore you already made kiddush midoraysa, then there is a chalos shabbos.
Indeed the shitos that disagree with R’ Akiva Eiger take a position similar to yours, that the person intends to make kiddush (and I add by extension – be mekabel shabbos) later, al hakos as the rabbonon were mesaken (e.g. Minchas Chinuch and others).
By a d’oraysa, when there is conflicting shitos one should be machmir.