Is it appropriate to drop by another university's professor's lecture to talk about research afterwards?

I would not appreciate that. I have a schedule, which means that after a lecture I may have a meeting, I may have an appointment with someone I am supervising, I may need to pick up my children from day-care, etc. etc. etc.

If you have emailed twice and not received an answer, then that is your answer... That is not very polite, but none of us are perfect.

No, that veers into stalker territory, don't do it. A lack of response is generally read as lack of interest. Even though she may just be busy or may not have seen your email, ignoring that convention, and going to these lengths to contact her, may ring alarm bells.

I feel your pain! We have all been in similar situations in which we have to guess whether the lack of a reply means "not interested" or "I just forgot to reply". From my 20 years of experience in academia I would say there is a very good chance it's the latter, but it could also be the former. Hence your dilemma.

What not to do: I would advise against trying to corner the professor after their lecture. While it's not exactly stalking it can be quite awkward if the professor's schedule is tight and has to go straight to another lecture/meeting etc. and has to brush you off as a result. While you could do it, there's a chance it won't work in the sense of actually getting to talk to the professor. And it may not create good will if done crudely.

What to do: There is in fact a commonly used way to contact the professor in person. Go to their office and knock! Even without knowing their office hours that is entirely appropriate. If the professor can't talk to you, they will say so. If they can talk to you, be prepared to schedule a meeting time rather than discussing your actual inquiry right there on the spot. A similar alternative is to find out their phone number and call them. Depending on your personal preference you may choose one over the other.

If you feel awkward about knocking at their door unannounced, remind yourself that they have had the chance to avoid having to talk to you in person by replying to your email. Given that is common for professors to forget emailing undergraduates back, you cannot be reasonably expected to interpret a lack of reply as a no. Good luck!