Lead Source Sync to Salesforce

We currently purchase lists for use in Salesforce and Marketo. The current workflow is flawed, and we are not sure where the problem lies in the sync. Here is what we are doing:

1. importing the list to Marketo, cleaning up the data
2. Exporting the list to Salesforce. During that export the "Lead Source" for each listing is established as the name and date of the purchased list.
3. Creating forms in Marketo programs (email campaigns and gated landers) that include a hidden field called "Lead Source."
4. Doing the twice a day sync to Salesforce, in the event that a Lead from a purchased list fills out one of these forms, the "Lead Source" appears to be overwritten

We would like the Lead Source field to check for a value in that field, and not include it in the twice a day sync for form completion. The problem is that we are losing information that we need to track regarding the viability of the lists we receive.

So, my question is, what is the industry best practice when it comes to Marketo and Salesforce syncing, both for initial batches of data AND for form fillouts that come in from Marketo? Are we doing something wrong, and what can we do to ensure that:

1. The initial "Lead Source" field in Salesforce never changes
2. People who are on a purchased list, so have a "Lead Source" already in Salesforce, do not have their "Lead Source" changed, but have the data from the Marketo progfram populate another field within Salesforce?

Thank you for helping us to understand this better!