Picking up from the question, Should you only update the source of truth? my thoughts are as follows:
Forcing users to pause using the current system and have them “swivel chair” or open another browser tab to make a change in another system is a bad idea. It breaks their train of thought, they may need to login to the second system, and they need to remember how to perform the update even though they probably don’t do it often.
All of these reasons create a really bad user experience and will really slow things down. This removes option 1 as a viable option.
Asking users to sometimes update adjacent fields to request an update in the Source of Truth breaks the user’s mental model of how Salesforce works. Imagine, the process would be something like: for all fields, update them directly. For this small list of fields however, click this button to submit a request for a change.
Again, a bad user experience. This removes option 3.
Thus, we’re left with option 2. I strongly believe that users should work in Salesforce as though it’s the only system they need to use. They shouldn’t need to wonder about other systems and how they are connected to Salesforce. Those details should be completely hidden in the background using automation and integrations.
The takeaway
The best option for, “Should you only update the source of truth?” is to have users update Salesforce as they always do, and have system processes in place to manage the rest. If the Source of Truth rejects the change, then rollback the update in Salesforce and consider notifying the user that made the update.