actually, joking aside, I think you have 2 separate considerations now. The room mix, and peace in the household (& neighborhood).
In any cubical room with flat surfaces, you only need to pad 3 of the 6 surfaces to keep the sound from bouncing around like the inside of a drum. Any 2 adjacent walls, and the floor or ceiling. So, I would consider an acoustical insulation panel on the wall with the guitars & the wall with windows, (& maybe an optional thicker drape during jams) If you aren't opposed to carpet, maybe that would be easier.. other wise, an acoustical insulation panel on the ceiling too & this will reduce bounce & you call play just as loud but hear less overtones & mud.
Now, for the rest of the house, in my opinion, you could absolutely go with solid acrylic panels on the wall over the stairs IF you pad the window wall opposite to it. It will not reflect the sound all that much differently then the other 3 walls have been all this time, PLUS you eliminate the back-and-forth bounce you currently have between floor & ceiling and between the entry & guitar walls.
On top of everything else, it might be nice to build a 3 part folding room partition just high enough for you or any other drummer to see over, & then pad the inside of that. This helps mic bleed a little, for casual recording, & where the other guys just turn a dial to bring it down, the drummer needs to play hard on the songs meant to be played hard & this helps the room mix without altering the playing style so much.
