Is don your new favorite word?1. Unless you stop someone in their car and ask them, you have no idea where they donned it or how many stops they're making. You don't need to don it at home, you don't need to doth it at home. You need to don before the first time you need it and after the last location.
2. Not sure what your question is. I didn't mention protection when discussing auto-infection. If you wear a mask in 3 or 4 stores, there's a possibility that mask has captured infectious material, from your own mouth or from the environment. In extreme cases, as zrickety has inadvertly pointed out, people not wearing or caring for masks correctly can end up with bronchitis, pneumonia, and other infections.