Can a person use footage obtained from a hidden camera as evidence in court?

If there is sound on the hidden camera, then the surreptitious recording of audio is a violation of California Penal Code section 632, which provides that the audio recording may not be used as evidence if either party was unaware of the recording. If there is no audio, while a surreptitious video recording of a person in a location where there was an expectation of privacy may be grounds for an invasion of privacy claim, it is nonetheless probably admissible if it can be properly authenticated. Good luck to you.

There are too few facts for a proper analysis. So long as authenticated and relevant, probably yes.

Probably yes, as long as it is admissible and foundation can be established. Be careful though, while video is okay, it may be a crime to record audio without the other person's knowledge.