In regards to multiplayer, we use the Photon service for the PlayFab Master Collection, there is a free version of it. The only difference is that the PlayFab Master Collection is a backend service implementation, such as for user data, inventory, currencies, friends, payments, cross-platform linking and cloudscript. For simple cases, like the features the multiplayer plugin has, the free one is already enough.
Yeah, the Mobile Master Collection have those tools. Our best ad-network mediation is the Mobile IronSource Collection, it also supports Google Admob.
I'm afraid we do not have in depth tutorials yet for all implementations, especially multiplayer. The current tutorials and documentation only focuses on each usage of the addons. In short, the addons are targeted for users who have intermediate understanding of Construct 3 and in need of professional features.
Feel free to check the documentation if you think it is suitable for your needs.
I hope that helps.