Our product Marketo Deduplicator (Marketo Duplicates: Leadonance Deduplicator to the Rescue – Marketo LaunchPoint ) allows to dedupe automatically thousands of leads from Marketo and pass these changes to SFDC using custom logic, so I can say about what happens when you initiate deduplication from Marketo.
1) If we have some sfdc fields hidden/unmapped from Marketo, how can we choose which info from those fields will be retained in sfdc?
Fields created in SFDC are automatically passed to Marketo, unless they are specifically hidden from the Marketo sync user, so if they have been passed to Marketo and if you specify no custom rules for those, then the value will be inherited from the winner lead. Let me know if I understood your question right.
2) Will all sfdc tasks and events be retained and merged?
If leads are merged in Marketo and they are properly associated with SFDC contacts and your Marketo sync user can change objects such as tasks and events, then all associated connections with events and tasks will be inherited by the winner lead.
3) What if two contacts are associated with different opportunities in sfdc, will the newly merged contact retain all those associations?
Same thing as with #2
Thanks, Paul. In regards to question one, we do have fields specifically hidden from our sync user.
As the reverse of this, if we have Marketo-only fields and decide to dedupe from the salesforce end, how will it choose which info to retain?