RSG –
“I presented this question to Maran Reb Chaim Kenievsky” ”
As with all recently “Maran Rav Chaim Q&A” ; they lack clarity, and I would never rely on any of those, unless written by a reliable Posek or by Rav Chaim himself. There are thousands passing and asking him all kind of questions and he replies in short. P’sak was always written with halachik/talmudic sources not just a short Yes and No. There have been some seforim like Hilchos Ketanos that were written as Halachos Pesikos and some opposed it. (which is a topic on its own)
“One may tell a person sitting in his makom kavuah to kindly move, even if there is not another seat available”
He can tell” now , is the person obligated to move? Is the person going to be offended? Is it before or in mid davening?
And lastly, can you or anyone explain where do we find “Makom Kavuah” to be a “right” on the expense of any other person sitting (as commented above)?
In any case, the very concept of “Makom Kavuah” is ONLY regarding Shmonah Esrei, thus he can only request the spot for that particular time, and the rest of davening he can take whatever is available.
Bottom line – if some late comer asks you in middle of davening to “move” because he has an issue with his Makom Kavuah” just reply you will move at Shmonah Esrei…