Is it likely cheaper to use a space heater than full central heating if you literally dont even use much of the space in the house that you never like go anyway?
You need to consider that some walls may contain water pipes. Not having those warmed up sufficiently could lead to a bursted pipe, which could lead to significant damage.
For us it was trial and error. We thought we were doing pretty well by keeping track of the weather and balancing the whole house heating vs single room. Until we had a snap freeze that plummeted the temps overnight to an obscene degree. We were comfortably warm in our bedroom as all the pipes in the kitchen and bathroom were bursting.
10's of thousands of dollars in demolition and reconstruction later we have decided that in this instance it's better to waste some small amount of energy keeping the whole house heated rather than risking another catastrophic failure.
Fwiw, most modern thermostats have an emergency failsafe temp setting that will always turn the heater on when reached, even if inadvertently set lower by mistake. Saved my bacon in a rental once.
This was a solid case of "if it ain't broke don't fix it" the thermostat and heating system in question was from the 60's or 70's and had served us with no issues for as long as we had been there. Hindsight 20/20 and all that jazz.
Fwiw there is low power heat tape you can buy to run along pipes to keep then from freezing. Had this on a pipe that ran through an attic and would freeze when the outside temp got extreme, despite heating the house.
Generally, 55-60 Fahrenheit (13-16 C) should be safe. If you have mild winters and a house layout that isn’t too spread out, you may be able to set it as low as 45 F (7 C).
That depends on the plumbing and how cold for how long it is in the area. Also, going too cold isn't good on the house, either. Drywall and wood don't like all the temp swings, either.