Let me give some insights to each one of your questions:
Currently, there is no built-in configuration in Datastream or BigQuery to selectively prevent DELETE or TRUNCATE operations from being replicated.
Yes, you have more control on your data transformation when you use the Dataflow pipeline into BigQuery. Feel free to browse this document for more information.
Besides Dataflow, another Google Cloud-native solution could involve using Cloud Functions triggered by Pub/Sub messages from Datastream. The Cloud Function would filter out DELETE/TRUNCATE operations and then write the remaining data to BigQuery. However, for high-volume data, Dataflow is generally more scalable and recommended.