The sync is one way: from Salesforce to Marketo. Updates to opportunities in Salesforce will be synced to Marketo.
The credentials you enter in Marketo for Salesforce are used to sync data over. Only data those credentials have access to will be included.
No, you can't. The changes to any opportunity in Salesforce will be synced over to Marketo automatically.
No, Marketo supports only one currency. The opportunity amount will be synced from Salesforce but the currency will be the default currency in your Marketo subscription.
Marketo associates Opportunities and contacts using Opportunity Contact Roles. Opportunities without any Contact Roles assigned will sync to Marketo, but won't belong to anyone. For example, the person won't qualify the Has Opportunity filter.
You can view a list of opportunities in the Opportunity Info tab in the Person Detail page.
Check out the constraints on filters and triggers. Lots of cool details in there.
Just make a new field in the opportunity object in Salesforce and it will automatically become a constraint!