Let me see if I can help you.
Marketo will sync the information back from SFDC as it is configured. Yes that does mean that if it is a contact, that the account details will be included.
Marketo will update the contacts as activity and interactions happen.
If a person comes from Marketo into SFDC, they will be created as a lead. Marketo does not have any Account matching capabilities for leads and is unable to convert a lead into a contact and assign to an account.
1: I would need to understand what you mean by "Operate out of the leads module" in order to answer this question.
2: No Marketo is unable to associate leads with accounts as this is a SFDC item. SFDC requires that contacts be connected to accounts.
3: I would need to understand what conversation visibility the sales teams want in order to answer this question as well.
I feel sales is worried about something that is a non-issue. By syncing both leads and contacts, Sales will still own Contacts and these contacts will simply have additional information added to their records such as page views, emails clicked on, marketing programs interacted with, etc. Come to the next User Group and we can go more in depth into your question.