Can a person have glaucoma and 20/20 vision at the same time?

Yes. As long as the central visual field is spared 20 20 vision is possible .
Yes. Glaucoma is a disease where the peripheral vision is usually first affected. Even a patient with extensive glaucoma damage , leaving only central tunnel vision can see the 20/20 line. Only if the central vision is lost with total optic nerve damage will the visual acuity be lost.
Most do. Most people with glaucoma have good vision. This is because glaucoma effects the visual acuity very late in the disease. However when this occurs, it is irreversible.