How do I allow my den, with door/window cut outs that face into the dining room, to double as a spare room? ?

1] do french doors with multiple panes - you can even get sliding [ pocket ] ones!

2] mount blinds above doors/windows, for privacy when needed.

Consider the folding door that slides out to close it off, after all if a person uses it as a spare bedroom, they will need privacy. That will allow you to keep the space open, and only close it when it needs to be closed in.

They make a nice variety now, so if you shop around you will find one that fits in with your decor.

As for the cut-out portion in the wall, consider decorate shutters that can be closed for privacy. You could hang them up and keep them open when the room is not used as a spare guest room. Plus, shutters are available in many styles, that way they will just show up on the one side of the wall so the other side is not changed.

I would definitely go with having pocket doors installed. They will allow for the space to be totally open when being used as a den and completely closed when being used as a bedroom. Because your existing wall already has pass through windows, it is not a load bearing wall, so converting it to pocket doors should be pretty simple! I'll be glad to give you more details if you need them. Can help you do it yourself or find the right contractor.

Hope this helps!

Scarlett