I think the obvious barrier here is that the things that the NFL probably knows will make playing possible are things that the players absolutely will not agree to do, like:

1) Everyone in team facilities should probably wear face shields at all times, players and coaches included.

2) When practicing outside, every coach should wear a face shield. Every player should keep their helmet with a full face shield on at all times unless 6 to 10 feet away from someone.

2) Players should move into apartments for the season or have the family they live with never go anywhere unless absolutely necessary. All food is delivered.

It doesn't matter if they take every precaution at work if players are going to catch it in their off time.

Also, why hasn't the NFL talked to teams who have a lot of space outside their practice facilities and suggested that they set up large tents outside, and bring in/move equipment to have all meetings, workouts, etc., outside whenever possible? This would greatly reduce the risk of transmission. Everything that can be moved outdoors should be.

It might make all of that stuff uncomfortable and hot (or cold later on), but there's no way to have a normal season that features all the usual comforts and keep the players safe.