The fast is not a commandment, like "You shall not steal." Breaking it does not damage our soul.
I think those are two different issues.
Indeed, we cannot say that following commandments will prevent us from sinning, but breaking commandments is sinning. Fasting is different, though. It is not a command, but it is good advice. We are free to follow it or not: if we follow the fast--the spirit more than the letter--then we will become more Christ-like. If we do not, we miss that opportunity. But never is the fast given the weight of a commandment; never is breaking the fast called a sin.
Now, if the people cooking Thanksgiving are aware of the fast and want to cook a lobster for you, please let them. If they are unaware and cook a turkey, please eat the turkey. Breaking a fast is preferable by far to breaking a relationship.
I understand what you're saying, and I appreciate it. My concern, however, has nothing at all to do with social events and eating what is provided. My concern is, instead, with lack of clear pastoral guidance.
Well, I'm sorry, but to get the sort of top-down, one-size-fits-all pastoral guidance you want, you're going to have to become a Catholic.