The issue with masks is that they do restrict gas flow to some degree when filtering the air that you breath. For the vast majority of people with no breathing issues this is not a problem at rest or during mild exercise as the tidal volumes that you breathe and the rate will be relatively low. With increasing amounts of exercise, it becomes more problematic, as you breathe more deeply and quicker. However even with moderate exercise, it’s the sensation of feeling a bit breathless exacerbated by the filtering effect of the mask slowing the flow somewhat rather than any appreciable change in either oxygen or carbon dioxide that kicks in first. That’s likely why those with existing problems with flow of air into their lungs, e.g. those with asthma or COPD, may find it worse with a mask, especially if they are doing any form of activity.
Having tested staff wearing the much more occlusive FFP3 masks, we know that in “normal usage” that body oxygen and carbon dioxide levels are not affected.
As for the return to grounds to watch games, I still think that even if this does restart, numbers will be very limited for some time (probably until we have an effective vaccine), which could mean that this coming season is pretty much a write off as well. As for when we might see full grounds again, no one really knows.