Cloud Functions only supports single concurrency, one request per instance at a time.

Cloud Run supports multi concurrency by default. You might see some benefit due to that difference (fewer cold starts, less CPU time allocated)

Yes, it needs to be specified upfront.

The default is 128M and you can specify up to 2G

Cloud Run engineer here.

Containers are run in gVisor https://github.com/google/gvisor as the container sandbox.