Can I set Windows default second-monitor behaviour to "Extend these displays"?

Also looking for a permanent configuration setting... In the meantime I'm using Iconoid (free) - it lets you save the icon positions for different screen resolutions, and you can set it to automatically restore positions when a screen is plugged in (new resolution is set)

With no second-monitor attached, open the charms bar and click Devices, then Second Screen, then Extend.

I've been having the same problem as you, and this seems to change the behavior for the next time you connect another monitor. I can't tell whether or not this changes the default behavior forever and for any non-specific second-monitor, but it seems to prevent the resolution on my laptop's screen from changing. I'd suggest routinely checking that before you attach any other monitor.