It will move with humidity and temperature change.

The electric boiler is basically a beefy, in-line hot water heater which, being in a closed system, does not have as great of a temperature lift from source water to output as a DHW inline heater would.If gas is more affordable in your area, then rather than ripping out all the existing supply lines and trying to install ductwork for a furnace - which can become expensive quickly, particularly if multiple levels or stories are involved - you could simply replace the boiler with a high efficiency (95% AFUE) gas fired boiler.

Today it as still cold, -20C (-4F) with a windchill effective temperature of -34C (-29F) and I, along with a crew of 3 others, spent the day clearing snow off roofs.

It burns at a hotter temperature and heats up the joints way faster.

Our electrical bill last month was $175 ... which included heat (the one baseboard) and running all our tools during 4-weeks where the daily high temperature was seldom over 0F.  

I'd be more concerned with 2 things:1) The drastic temperature change the house will experience in going from white to black shingles2) A home inspector for your end buyer who might throw up a red flag since he won't run into something like that very often.

If there is/was little insulation in the attic (the ceiling joists are exposed above the insulation), you would have cooler/warmer temperature along the bottom of the joists which could both cause moisture issues and may have affected how the smoke particulate stuck to the ceiling.If you take the pointed tip of a knife or {one blade} of a pair of scissors and {gently} press into the drywall along one of those lines, is the drywall hard or does the knife tip slid in fairly easily?  

It also loses its R value as the temperature decreases.

I also have WiFi thermostats so I can remotely change the temperature when no one is there to save on the electric bill.

SF Bay Area, especially in terms of SFH construction quality, market temperature, tenant quality etc.