It's OK to assign the new lead to a user - just be aware that a SFDC Lead record is being created - not a Contact record. All your data associate needs to do is to convert the Lead to a Contact and associate it with the appropriate Account (or create a new Account during the conversion). This method would prevent data entry errors by your data associate creating the Lead manually and eliminate the resulting duplicate Marketo lead record. You would need to tweak your alert email to the data associate to include a link to the SFDC Lead, which you can provide by including the token {{SP_Send_Alert_Info:default=edit me}}.
What do you do with the duplicate records - do you merge them in Marketo?