You can only file for annulment AFTER divorce is finalized, so of course, after that you would file taxes as single.
There are two types of annulments for Catholics, A Declaration of Nullity and A Formal Case. The former is filed if AFTER the marriage took place one partner refused to have kids, was unfaithful, abusive etc. The latter is filed mostly with people who are not Catholics.
According to the Catholic church: a church annulment does not imply that the marriage never existed, but only that it did not have the character of a sacrament (sacrament: the couple must understand what marriage is and they must intend their marriage to be a lifelong partnership which is open to children. They must intend fidelity and the mutual good of one another.)
I do not see how this could affect one's citizenship. Any comments anyone?