I am not so sure they need 100% to be at least 2 plants, I think this was discussed here in other topic. But growing only one plant will not give you enough fruits to make a salsa verde, unless you harvest some, keep them till other ripe, and so. Or you make a small quantity of salsa verde...
They are ripe when the fruit will fill the husk (initially the husk looks empty), sometime the husk will "crack". Not so familiar with tomatillos, I am growing them in the last 3-4 years in my garden, but mostly empiric, so to say. When over-ripe, they became pale green, a bit yellowish green, and later they crack too. I am using mostly for salsa verde, but can be eaten fresh too.
No idea if eating them unripe, but why to do this? However, I used them also a bit before ripe completely, and had no problem