Noble families start from somewhere, and BM has lots of new noble families. Some people play those new noble families as just being minor families now being more prominent, but others RP it as the founding of a new noble line, usually for notable actions. That nobility did not extend backwards (as far as I know) to your mom/pop, grand-dad/grand-mum, great-grand-dad, etc. So it's quite conceivable your mum might be a commoner (also see above about wealthy merchants).
Whatever people state in their RP, the official game explanation for all "new" noble families is that they just weren't prominent before.
Adventurers become nobles all the time.
Adventurers prove their pre-existing noble heritage all the time. They do not change from being commoners to being nobles: they change from being unrecognized nobles to being recognized nobles.
Nobility was recognized by blood and by deeds. They key thing is that you either have inherent or demonstrated Superiority, which explains why you are fit to lead commoners.
The key thing is that you have noble blood. It doesn't matter how stirring a speech a commoner is able to deliver: he's still a commoner.
