Quick check here:
If I have a field called Marketing Group on the account object. The same field is on the Contact Object, as a formula field to read from the account field.
A flow step from Marketo will NOT be able to change the value of that field if the record is a contact, correct?
If the record is a lead, the flow step can change the value. However, what happens when the lead is converted to a contact? For the scenario where there is an existing account and for the other scenario when it is a new account?
Thats an interesting question. I think the Account data would trump the lead and contact information being sent over as Account data is not by directional between SFDC and Marketo (SFDC wins that battle). If its a new account I think the data might pass from the Lead to the Contact and then up to the Account. I think the best way to know for sure is to test it. Let me know what way this plays out please.
From the docs : SFDC Sync: Account Sync - Marketo Docs - Product Docs
"If you update an Account field for a contact in Marketo, it changes the values of all those contacts belonging to that account in Marketo. It does not sync to SFDC. However, the next time that account is updated in SFDC, the changes will override all account information in Marketo. "