If I create a user profile temporarily for specific activities that I don't want to keep records of (in case I'm ever forced to divulge my phone's passphrase) and then delete it, does that remove any chance of recovering information about the activities on that user account through forensics, even if I give my main user profile's passphrase?
Does deleting user profiles defeat local forensics?
Yes. Each profile is separately encrypted with its own keys - once it's gone, it's gone. See the faq for an in-depth explanation:
spring-onion once it's gone, it's gone.
Data is gone, but the fact that a profile has existed may persist.