I may be wrong, but I think it's all about the "quality" of the preposition. E.g. the preposition by (plus some particular time) demands that the verb describing the previous action should be in the past perfect.
In 1a) the action had been completed by 7, so the wife didn't see him washing his car while in 1b she might have come in to see him washing it.
For 2a, 2b, 3a, and 3b my explanation is the same (correspondingly).
I think one can't use until in 1a, 2a, and 3a the way you wrote them.
But I am also eager to see what the native speakers will say about it.
Student or Learner