As for Firestore, it’s not clear whether it supports cross-collection transactions. Cloud Datastore does not support cross-namespace transactions AFAICT.
b) Given that the primary use case for namespaces was/is multitenancy, it's not clear to me why you'd want to transact across them. Nevertheless, you can. What's leading you to draw this conclusion?
The documentation is what led me to that conclusion, since it's not explicit as to what the transaction boundaries are, but I could be mistaken. Does this mean that the poster's claim is erroneous?
b) Given that the primary use case for namespaces was/is multitenancy, it's not clear to me why you'd want to transact across them. Nevertheless, you can. What's leading you to draw this conclusion?