Learn English – what do you call someone who always wants to dress up nicely

The phenomenon you speak of has a distinct studied name which I remember from a psych class from long ago - ad hoc hypothesizing. If I remember correctly it is when you compensate for abnormalities or issues by creating false rationalization(s) about a topic - in basic terms, changing your reaction based on outcome. I would just call them a liar. These people are certainly pathological.

The thing about ad hoc hypothesis is that they are stated in a way that is almost impossible to prove wrong. In your example Mary states she purposely left the food in her teeth. It is almost impossible to discredit this.

Now if the person is just doing it on purpose to be funny they would be a smart-ass.

Also for a person who is kind of in-between the liar and smart-ass, the person who change an argument based on the end result would have circular logic. In your specific example this could work, but we know that 99% of people don't leave meat in their teeth on purpose.

Consider that fine word obscurantist (“A practitioner of obscurantism; an obscurant”), where obscurantism has a sense “being deliberately obscure or vague”, and obscurant as an adjective means “Acting or tending to confound, obfuscate, or obscure”, or as a noun, “One who acts to confound or obfuscate; an obscurantist” or “A person who seeks to prevent or hinder enquiry and the advancement of knowledge or wisdom; an agent of endarkenment”. The latter phrase also suggest the term endarkener.

Note, by the way, that “security through obscurity” (a general term for keeping things secret by hiding them) often is denigrated as a security policy.