Hi all--
I think I may have found a viable solution for this.
To refresh, the main issue here is being able to not only have Lead Score be a sum of both Demographic and Behavioral, but also be able to update/edit the individual Demographic and Behavioral scores and still keep Lead Score equal to the sum.
I've set up a Workflow rule in Salesforce that adds Demographic + Behavioral to equal the Lead Score, everytime either Demographic or Behavioral score changes. This gets around the issue of changing the Lead Score field to a formula field and removes the need to create an additional "Lead Score" field.
The main benefit here is that it allows me to clear out Behavioral Score from the Lead Score without having to remove the Demographic score from the Lead Score because now, I can set up a recurring campaign that rescores Demographics every day.
The downside is that there will be some lag time. Depending on the timing of events, that lag time can be as short as 5 minutes (the regular Marketo-SFDC sync), or at most a day depending on when you've set the recurrence to occur.
I hope this helps anyone who's also struggling with this. Let me know if you need any clarification or if you think there are any potential issues with this solution.
Thanks,
Hoi