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.

Well, you can try this trick. Create a text file in notepad and name it, lets say "ExtendScreen.reg. Then open it in notepad and right the following:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run]
"ExtendScreen"="\"C:\\Windows\\System32\\DisplaySwitch.exe\" /extend"

Then save it. Of course if windows is in other driver than C: then you have to change it. Or you can use the following value instead of the previous one:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run]
"ExtendScreen"=hex(2):25,00,57,00,69,00,6e,00,44,00,69,00,72,00,25,00,5c,00,53,\
      00,79,00,73,00,74,00,65,00,6d,00,33,00,32,00,5c,00,44,00,69,00,73,00,70,00,\
      6c,00,61,00,79,00,53,00,77,00,69,00,74,00,63,00,68,00,2e,00,65,00,78,00,65,\
  00,20,00,2f,00,65,00,78,00,74,00,65,00,6e,00,64,00,00,00

This is basically the same value but it's using Expandable String value instead of String value. The value is:

%WinDir%\System32\DisplaySwitch.exe\ /extend

This will be resolved every time windows boots up. This should force Windows 7/8/8.1/10 to select always Extend Desktop. All you have to do after saving that text file is to double click on it, or select it then hit Enter key. Windows will ask your permission to merge it with Windows Registry, just click on "Yes" button, then click "Ok".