“can a shabbos goy do the replenishing for an individual on shabbos?”
It is permitted to ask a goy to turn on the heat since it is necessary for health and gzeiras amira l’nochri did not include heat.
The entire idea of Shabbos goy comes from this. Every Jewish house or building had a goy who would come in every Shabbos to take care of the fire.
“jews who lived in isolated cold climate with no nearby neighbors”
The only Jews I could think of were those in Siberia. No, they did not keep warm. That’s how they died.