Dynamics 365 Debug Custom Workflow Activity in a Real Time Workflow

 You CAN'T.

If you are having trouble following the steps to debug your custom workflow activity because the steps are not appearing, it's probably because you cannot attach to "Real Time" workflows.  If possible, make your workflow asynchronous (run in background) to debug your custom WF activity.

Email Templates for Custom Entities

I have used the trick of inserting the "magic" sequence of characters that allow you to insert custom entity fields into email templates, but there's an extra special trick if you insert a lookup field /@name.  Big thanks to Insight Dynamics blog post for the tip.  Here's an example for an entity called "Custom Entity":

{!new_customentity:ownerid/@name;}

UCI Email PartyList error - Entity type "queue" is invalid

I just ran into an error where some clever code check's the user's business unit and defaults the FROM addresss of an outgoing email to a particular Queue.  The code started throwing this error once we moved it to a Model-driven App.

It turns out the error was occurring because we had not included the Queue entity in our Model-driven App.  Once we added the Queue, the code worked just fine.