I understand that if I hear another person make a bracha (and perhaps even if I don't actually hear it), I may? must? respond "amen". I also understand that saying "amen" can allow me to be yotzei (fulfill my obligation) through that other person's bracha, so, e.g., we don't all have to make kiddush individually. I understand that the speaker has to have some intention to include others in that latter case.
My question is about how these interact:
Suppose I overhear someone make a bracha that he intended only for himself (e.g. he's about to eat something). If I say "amen" am I just being polite, am I fulfilling a mitzvah even though I get no benefit from the bracha itself, or do I now have to eat?
Suppose I overhear someone make a bracha that he intended to include others in, though not necessarily me specifically (e.g. motzi at the next table). If I say "amen" am I now yotzei through him because he intended to include others? What if I don't want to be yotzei through him?
Suppose I hear someone who intended to include me make a bracha. If I say "amen" with the intention of being yotzei, am I? What if I don't want to be -- do I say "amen" and I'm not, do I not say "amen", or am I yotzei whether I want to be or not?
Not wanting to be yotzei in these last two cases could be either because I've already made the bracha myself (e.g. I lit my own chanukiyah before joining friends) or because I intend to make my own bracha later. So either way, I want to avoid a bracha l'vateilah.
When does saying "amen" make me yotzei, when does it not, and when does it give me the option?