In other words – and I think that this addresses Wolf’s question – There is a difference between being mekabel tosefes shabbos l’inyan issur melacha (after which one can still daven Mincha, according to SOME poskim), versus the full 100% acceptance of kedushas shabbos (which would preclude davening Mincha later).
Mik, once again, thank you for the cites. However, both RSZA and RMS are *very* modern sources. On what did they base that there was this possibility of “splitting” Shabbos up like that?
The Wolf