Would native speakers of Romance languages be able to understand + reply back to people who speak Interlingua?

YOU MEAN INTERLIINGUAL OR BILINGUAL

Chances are, they'll be able to understand Interlingua (I can understand Interlingua pretty well), but responding is a different story.

Responding in a language requires that you know the grammar of that language, otherwise your response would be littered with the grammar of the Romance language(s) that you know.

For example, Interlingua uses "le" to mean "the" for all nouns. This is a concept that's very strange compared to the rest of the Romance languages, which have at least 2 different genders. So you might get natives slipping in a "la" here and there.

Most likely, yes. Since Interlingua was built around Romance languages, most likely, it would be possible to understand Spanish, French, Italian, Portuguese, etc. It might even be possible to understand Ido and Esperanto, but here's the thing, I wouldn't learn Interlingua if I were you. Since it is an auxilary language, no one speaks it like a native. So, I would rather you learn a real language (a real Romance language). If you learn Spanish, most likely you can easily learn Italian and Portuguese.

Hope I Helped!

XD