VPC Network Peering allows internal IP address connectivity across two Virtual Private Cloud (VPC) networks regardless of whether they belong to the same project or the same organization.
-> Shared VPC is only within an organization - it allows an organization to connect resources from multiple projects to a common Virtual Private Cloud (VPC) network, so that they can communicate with each other securely and efficiently using internal IPs from that network.
-> Private Google Access is only to access Google APIs and services
Reference:
-> https://cloud.google.com/vpc/docs/vpc-peering
-> https://cloud.google.com/vpc/docs/private-google-access
-> https://cloud.google.com/vpc/docs/shared-vpc

The PAYG model is more convenient because you only pay for usage. And the case describes that the workloads are only run on certain days.

This is the definition of a least privilege model.
A supporting principle that helps organizations achieve these goals is the principle of least privilege. The principle of least privilege addresses access control and states that an individual should have only the minimum access privileges necessary to perform a specific job or task and nothing more

https://cloud.google.com/resource-manager/docs/creating-managing-folders

Explanation
Logged information
Your Google Cloud projects contain only the audit logs for resources that are directly within the Cloud project.
Other Google Cloud resources, such as folders, organizations, and billing accounts, contain the audit logs for the entity itself.
Graphical user interface, text, application Description automatically generated

Reference link- https://cloud.google.com/storage/docs/audit-logging