Anyone have any experience with cafe-cabins near the forests?

The concept of these sounds cool but weird to me, as someone from the UK.

They are very common in Norway. Even modestly sized villages often have one or two places like this.

Are they almost like small community centres situated in strategic places so to serve hikers and people from the surrounding community?

Yes, and especially skiers. These places are more busy in the winter than summer.

Am i correct in believing i can treat it as any other shop for baked goods and hot/cold drinks?

Yes.

Rustadsaga seems (from their webpage) to have a real selection of baked goods. Smaller places may only have waffles (unless they in svele territory). In places like this you'll always get coffee, hot chocolate, hot or cold "saft" (diluting juice). You'll typically not find an espresso machine in small places like this, only traditionally brewed coffee. (Rustadsaga is so big and close to Oslo; they may have retrofitted an espresso machine.)

Places like this are decent if you are hunting for traditional Norwegian food. I see Rustadsaga serve lapskaus (A stew with meat potatoes and swede) on Sundays. Other typical options for a meal you might find in places like this are rømmegrøt, spekemat (google for images), rakfisk, karbonadesmørbrød, raspaball/komle/kompe and hot dogs.

It seems like the opening hours are really short but it's so close to Nøklevann which is where we're going to chill.

Smaller places like this (in rural areas) may only be open Sundays 12-16, possibly completely closed spring/autumn.

Rustadsaga seems to specifically cater to swimmers/sunbathers at Nøklevann.

Don't be afraid to hike or bicycle on the paths to other places like this in the area.

One other activity this time of year is "tur-orientering", essentially an old fashioned form of geocaching. You can probably buy an envelope of maps and instructions at Rustadsaga if you want to try it out.