Yes. Containers will be enabled by default in the regular release without the need to install the extension or flip a preference in about:config.

It's also planned to integrate the ability to assign a website to a specific container. There's no information available on the other extra features from Multi-Account Containers like VPN/Proxy per container for the moment.

You can follow the development on bugzilla https://bugzilla.mozilla.org/buglist.cgi?component=Container...

It seems so, especially because they explicitly say in the help page[1] that "Multi-Account Containers" brings "a few benefits on top of the native container functionality like site-specific container assignments, syncing across devices, and integration with Mozilla VPN or your own proxy".

[1]: https://support.mozilla.org/en-US/kb/how-use-firefox-contain...