Will just turning on the car without driving prevent battery damage?

Unplugging the 12V isn't difficult, but if you go that route remember that there will be no power to the door and hatch locks. To hook up the 12V again you would need to open the driver's door with the physical key in the fob, crawl into the trunk area, pull the manual release (which is behind a plastic door, unless somebody has pulled it off and thrown it away). Alternatively, after opening the driver's door open the hood, and put 12V on the jump point long enough to unlatch the hatch. If the boyfriend doesn't drive I'm guessing you don't have a source for that 12V jump.

If the boyfriend starts the car periodically be sure it stays in Park. Don't bother revving the engine. The car doesn't have an alternator and so, unlike a conventional ICE vehicle, it will not charge any faster at 4000 RPM than it will idling at 800 RPM. In a driveway I might leave a fob locked in the car and come back in an hour with a second fob to open it, but in a public garage it is probably better to stay with the car while it is on.

Be aware that the car will mostly be charging the 12V from the larger battery pack, and the motor will only run once the pack has run down quite a ways. This can be a little disconcerting for somebody not used to the car, because it will seem like the car has stalled, but this is normal operation.

 

Well, you shouldn't do it that hard-to-reach way. You should disconnect it at the fuse box under the bonnet. There's a connection there for that purpose, same as there's an easy-to-reach jump point.

 

The "disconnection" on a US car involves removing a hold down nut on the live 12V post, and then pulling the substantial cable (which is live) up and off. Then putting it someplace "safe". It would be all too easy to accidentally short to ground using that method. Personally I wouldn't touch that cable unless the ground cable was off at the battery.

Perhaps the cars in Finland have a safer disconnect method.

 

You might also worry about the catalytic converter being stolen.