Hi Isabela,
To answer your questions:
1. Yes, just make sure that sales insight appears on the contact view/layout in SFDC
2. This would potentially be something to test, but I'm most comfortable with doing single-person sends, and recommend that our users do the same thing.
3. Yes, the sender information goes back to the SFDC user who sent the email. If you want to have an email go from someone else, you can use request campaigns that trigger an email send from Marketo that references someone else's send information.
Hope this helps!