if you want to use the adverb you can use it for spending time we can spend our time comfortably in our armchair.

Yes, you've put the adverb "comfortably" in front of the noun, where the adjective should be. The armchair is only the location. Put the adverb closer to the action! Try, "we can spend our time comfortably". If you want to talk about the armchair, use the adjective "comfortable".

Yes, that is incorrect. We can spend time in our comfortable armchair. OR We comfortably spend time in our armchair.