Indeed only one metadata schema allowed per BluePrint hierarchy.
BluePrint hierarchy in this case means standalone tree.
You can change metadata using event system
By subscribing to "Item Send" event:
EventSystem.Subscribe<ITranslationJob, IItemSendEventArgs>(OnItemSend, EventPhases.Initiated, EventSubscriptionOrder.Normal)...
Customer should use Event System approach.
Old Translation Manager Plugin system is deprecated for many years.
No. Minor version translation deals with source item being in workflow.
Review functionality works with workflow on target item.
All you need for Review functionality is workflow on target items, that supports Translation Manager Review. And appropriate workflow steps on TMS or WS.
When I've set Translation Review up in the past the AllowTranslateDynamicContent setting was not enabled (so left commented out in the TranslationManager.xml file as per default) and Translation Review worked as expected.
<!-- Enable translation of minor versions. -->
<!-- <AllowTranslateDynamicContent>true</AllowTranslateDynamicContent> ...
I think the documentation needs updating. Looking in the Installation Media, theres now an EventHandlers folder with a ErrorEmailNotifications.cs file which using the Event System and not the old plug-in approach.
public class ErrorEmailNotifications : TcmExtension