Hi Karen,
1) There was a recent discussion on this topic. Please see
here. You can't import using the Lead Owner field directly, but there is a way around it.
2) Marketo will de-dupe at the email address level. If the email address of the person in your list matches an email address in SFDC (as a Contact or a Lead), the record will be updated and a duplicate will not be created.
3) I usually have a Trigger listening for the list import to change various fields like the Lead Source and Lead Status if the field is empty. This will only change the data for new leads.