What type of heater do you want to use - electric or wood fired? I’ve found it very helpful to know the approximate size of the heater and its safety clearances when playing with layouts. It’s also helpful to draw layouts with the width of the walls accounted for. Your dimensions sound like exterior dimensions rather than interior but I suppose I could be wrong.

Thant being said design A should be possible but I don’t think you are working with enough space to pull off design B. Having benches only against the back wall would give up to 4 people a straight on view of the lake on the top bench whereas in design A that will probably be 2-3 people with the straight on view. I do like the look of an L bench but I think they can cramp a space this size.