Recently a very large person, walking with a cane, boarded the subway. While my first instinct was to offer my seat, that seat alone would not have accommodated this very large person, and the adjacent seat-holders were themselves elderly and/or infirm. So although offering my seat might seem, at first glance, the gentlemanly thing to do, it would force otherwise deserving passengers out of a seat, or put the very large person in an embarrassing situation. What should I have done?
Etiquetteer must commend you for your discerning observation, and also for your concern for others on public transportation. How many times does one see the elderly or the pregnant standing with difficulty in a train full of the selfish and the oblivious?
Your summary of the situation indicates that you're "damned if you do and damned if you don't." Whether you keep your seat or surrender it, someone will be inconvenienced. Etiquetteer can only offer you the Lady Sylvia McCordle Solution, which she so elegantly presented in Gosford Park. At the least sign of trouble, just walk away. When Mr. Weissman offered to reverse the long distance charges, when George spilled coffee in the lap of that impostor, when Lady Sylvia couldn't honestly return a compliment about Mabel's evening dress, she just walked away.
In your case, Etiquetteer suggests you leave your seat and move into another car. This way you needn't linger to observe the outcome.
For those of you on Facebook, Etiquetteer has set up a fan page, from which he links to and comments on articles in the news media.