When you do not know the actual count of people who have the virus, due to the fact you admit the tests you are administering can indicate a reading of positive even if you have another type of coronavirus, such as those that cause colds, I fail to see how you can possibly determine how it is transmitted.
Does the test need to be 100% accurate to conclude that the virus is spreading among the general public? There may or may not be a perfectly accurate count, but all of those people who are filling ICU beds in Florida, Texas, California and most of the rest of the country/world had to get the virus from somewhere. Where do you think they got it?
Why is masking in public not a good hygiene practice?
Because it has deleterious effects on users, requires frequent adjustment and touching of the face, and only sick people should wear them.
To which "deleterious effects on users" are you referring? If a mask makes it hard for someone to breathe, then that's a pretty good sign that they probably already have some underlying condition that makes getting covid 19 an especially bad thing.
Also, what about the pre-symptomatic/asymptomatic people? Should they wear masks to help reduce transmission of the virus?