You should have a report that is a blocked field report, so we can see all the fields and if they are blocked and for what reason.
It is rather cumbersome to go through each field and click them to see what is block and by what.
Also, you should be able to have a "fill in blank" option, where data from a list load or source into Marketo would fill in the blank field in Marketo (and thus SFDC). Then you could always add new data, as you get it. While protecting any data already there.