Because of your switch, this field is now controlled by SFDC. You need to change owners and those fields will be automatically populated with the owners information in SFDC.
I think you are over-complicating this. It is always advisable to use one field to hold Account Owner and now that you have Salesforce syncing (which btw. is pretty awesome), this should be left with Salesforce to control the native Account Owner field.
What you can do to get around this situation is create additional accounts on Salesforce for fake users (might be expensive) and have the emails sent out with their names (warning: not a best practice as prospects look up on the internet); or always assign the Accounts to SDRs or Sales reps and let the emails go out in their name. If there is an email which should not carry a sales person's name / email, then sending that email addressed by your company name and general sales email (eg: email@example.com) should do the job.
If the business case requires using fake names, you can still create two additional fields on Marketo (let's say Marketing - Account Owner & Marketing - Account Owner Email) and copy the values for accounts that belong to sales reps (using conditional change data value step) and the ones owned by Marketing can have different names / emails. These fields will go as From Name & Email on your email campaign.