GH: your comment “although most MO shuls only “sell seats” for the yamim noraim” is where most of the misunderstanding is. Young Israel’s, in general, have different rules about seats. My grandfather davened in a Young Israel and sat in one seat on the right hand side of the shul (for ~ 22 years) except for the yamim noraim when he “purchased” his seat on the left side of the shul that was assigned to him when the shul was built.
In the smaller shteebles (shteeblach), seats are generally sold for the entire year (generally from Rosh Hashana to Erev Rosh Hashana) so this way shul members will have a place to sit when they come to shul. I davened in one shul for 11 years, purchasing the same seats all 11 years.
IMO, the whole thread can be boiled down to the essential questions:
Does the guest/newcomer have the responsibility to ask about a seat or is it the mispallilem who have to look out for the guest/newcomer?
Does it make a difference if it is before the scheduled time for davening or after davening has started?
Does the shul have a rule about seats and are the rules posted for everyone to see?
Does the shul have the seating chart posted in a conspicuous place so that the guest/newcomer can check for himself what seat is available?