Teams need to conduct practices in their own facility, not somewhere miles away because the players will need to be tested periodically and kept out of too much public contact while their job is to go from home to team facilities and train. This is why NFL wants to have no leniency in allowing any outside facilities.
So, teams conducting any practices outside their facility will have to change and accommodate this Off-season. Strictly.
NFL has given a direction that "teams can get Exception but they will have to prove to the league Medical Task Force that they can't conduct in their own facility" if that's the case.
I think that would mean team would've to move their facility to their outside training location so that medical tests and player frequency is limited to that outside facility. Not sure if teams could do that or it would be prudent to just use/upgrade the practice field and training rooms in their own facility to conduct off-season program.
By September, it's more likely that stadiums are allowed to have fans in limited or very limited capacity, a few allotted seats in safe distance in each section.
Stadiums will not have 60,000 fans but it's more likely that 5,000-10,000 fans can be accommodated keeping the distance as per state orders. Would that be a boost for the home team? Yes, better than having no one. Would that create a noise that gives trouble to visiting team on the road on important downs? Most likely not.
Maybe, no tailgating and things as such or that in limited capacity. Maybe, it will improve throughout the season but I doubt games will be played with no fans.
Probably the opponent traveling fans wouldn't be there much as usual, unless teams in proximity to each other play.
It will be an interesting season, but it will not be gloomy without any fans at all.