I do see your point, but I wonder if we can take those snapshots of Snape's memory as representative of his whole childhood. Particularly in the case of the second memory - what teenager hasn't sat moped in their room, miserable with boredom and perhaps a little angry about their lot in life?

I'd agree, except that those two memories seemed to be right on the surface of Snape's mind, as it were -- the first things Harry saw when he looked there -- which to me suggests that they were characteristic or representative of Snape's childhood and adolescence as a whole.