“Well, the Mishnah Brurah says that it means the man’s seat in shul, but that anywhere he sits within daled amos of his seat is still his makom kivua.”
Furthermore, The Mishnah Brurah does not imply on the account of another person already davening, you can offer the person another seat, but the inyan of Kivua does not override anyone’s right to have a seat or to embarrass someone.
The very same Shulchan Aruch says that prior to davening you should be mekabel mitzvah of ‘ואהבת לרעיך’ etc. And also says “יקבע מקום לתפילתו שלא ישנהו אם לא לצורך – a guest sitting in your seat might very well be classified as “לצורך”, if there is no other seat to offer or can’t be done in a proper manner.
Additionally, there are Rishonim who hold that makom kivua only applies to your house, although we don’t rule like that, but I’m sure you can apply it to all the above, depending the situation.