A parent who knows a second language with an accent is trying to teach it to their child while living somewhere that the language isn't the dominant one spoken. However, s/he has access to native speakers. There are radio stations where the language is spoken, TV channels, access to DVDs, CDs, and/or the internet. How much exposure to these media is needed for the child to pick up a native-like accent? How much exposure to these media, versus the parent, is optimal for overall language development?
Same as above, but the parent has a friend/coworker/etc who natively speaks the language (in addition to access to the various media).
Edit- Scenario #3:
The friend/coworker in scenario #2 has children close to/slightly older than the parent's child's age.