Yes, if Obama was born abroad to a bigamist father, Ann’s citizenship would confer a citizenship on him, even though she was under 18. But this would be citizenship by statute. He would not be a natural-born citizen.
Barry would not be a “Minor v. Happersett NBC” but only if MvH is upheld as applied to the facts and circumstances of Barry's birth.
Note that Barry's legal team have now moved way beyond MvH, WKA and Ankeny to the new standard articulated in Marguet v. Pillado (9th Cir. 2011) which says that Barry would be NBC regardless of where he was born on the planet as long as he is biologically related to a US citizen at birth.
See my FR vanity thread:
http://www.freerepublic.com/focus/bloggers/2857598/posts?q=1&;page=101