". . .I'll have sidewalls but no front and back walls. Would that be a problem?"
The side wall lateral reflections are beneficial - particularly in stereo - since they can produce greater apparent source width and spaciouness, while the front/rear reflections can decrease these attributes since they come from the same direction as the direct sound and thereby increase the Interaural Cross Correlation Coefficient (IACC) which is a bad thing. So in short the situation of having side walls with no front/back wall is preferred over the opposite situation. Have you considered mounting some diffuser panels (DIY or store bought) on a moveable stand with wheels that can be moved to the side out of the way when not in use or put behind your listening chair for critical listening sessions? Just a thought.
"Why do thin windows act as an absorber of bass?"
Windows don't technically absorb bass unless they're loosely installed and their vibration back and forth acts like a diaphragmatic bass trap . . . As low frequencies have long wavelengths they don't "see" the window and easily pass through it; if they pass through it then they can't be reflected back into the room so therefore the windows function more like a bass trap/absorber. Higher frequencies with much shorter wavelengths will in fact "see" the window and reflect off of it. The biggest problem with the windows is that it's too good of a bass trap and allow too much bass energy to be drained out of the room while maintaining the higher freq energy and therefore acts like a high-pass filter throwing off the timbral balance which you hear. So diffuse or absorb in front of the windows to attenuate the higher frequencies.
I don't know your ceiling height but you could experiment with reflection, diffusion, or absorption at 1st reflection points there to see which you like best.