Stack ventilation is designed to produce 100 cfm for a room with 2 person and volume 2400 cu. ft. How much temp change is possible inside the room when air at temp 81F flows through the room at this flow rate...Is it enough to create comfort without mechanical HVAC systems??

You didn't say anything about sources of heat gain, but let's assume they are minimal. I don't know, are you comfortable at ~82F at whatever humidity you typically have and no noticeable air flow? Consider adding an efficient ceiling fan for some evaporative cooling. Or a whole house fan if you want evaporative cooling air flow in all the rooms at less installation expense than so many ceiling fans.