Note: You can scale the number of nodes in your instance based on the Cloud Monitoring metrics on CPU or storage utilization in conjunction with Cloud Functions.
https://cloud.google.com/spanner/docs/instances

An external data source (also known as a federated data source) is a data source that you can query directly even though the data is not stored in BigQuery. Instead of loading or streaming the data, you create a table that references the external data source.
https://cloud.google.com/bigquery/external-data-sources

https://cloud.google.com/spanner/docs/getting-started/set-up

A is not correct because Cloud Datastore is not the most performant product for frequent writes or timestamp-based queries.
B is not correct because Cloud Storage stores blobs, which cannot be queried.
C is correct because Cloud Bigtable is the most performant storage option to work with IoT and time series data.
D is not correct because although it can store the data, BigQuery is very slow at changing data.
https://cloud.google.com/bigtable/docs/schema-design-time-series

Changing the service account and access scopes for an instance
If you want to run the VM as a different identity, or you determine that the instance needs a different set of scopes to call the required APIs, you can change the service account and the access scopes of an existing instance. For example, you can change access scopes to grant access to a new API, or change an instance so that it runs as a service account that you created, instead of the Compute Engine default service account. However, Google recommends that you use the fine-grained IAM policies instead of relying on access scopes to control resource access for the service account.
To change an instance's service account and access scopes, the instance must be temporarily stopped. To stop your instance, read the documentation for Stopping an instance. After changing the service account or access scopes, remember to restart the instance. Use one of the following methods to the change service account or access scopes of the stopped instance.
https://cloud.google.com/compute/docs/access/create-enable-service-accounts-for-instances