The "missing field" sync error normally should not prevent the sync to happen.
Unless that field was required in SFDC to create a record or do something, which I'd hope it wasn't!
If you are no longer using the field, you should rip it out of Marketo:
- delete all values in Marketo
- remove field from all assets
- ask support to delete the field
But otherwise, it will just throw that error. The only time the entire record will be blocked is if SFDC could not get the correct value for a required field or a cross object error occurred. Did you check to make sure the rest of the record updated in SFDC?
The initial error DID prevent the sync but it appears it was at the precise moment the field was deleted from Salesforce, before it was hidden in Marketo. When I tried to re-sync the leads went through.
On cannot delete a field in SFDC if this field is used anywhere (Appex, workflow or validation rule). So you do not have a risk here.