I wouldSign the lease with the ABC, have C sign a forfeit of lease when they go, have D sign on original lease when they arrive.

We wanted to move before our new born arrives in early May, but that isn’t looking likely at this point.

Intercoms with the door buzzer thingy.Especially for young people because:A) - they hate to go out and buy stuff they prefer it all arrives at their door in a big brown truckandB) when their stuff does arrive in the big brown truck they hate just a much having to answer the door or go down the stairs to let the guy in and to retrieve their stuff.They basically want to be able to order bubble bath from their bath and have it delivered right to them there in the tub.Adding the intercom/buzzer system if not already there does justify an nice rent bump come renewel time.

Should I let the tenant know right after or before I leave (arrive at) the closing table?

Tenant arriving at 6 pm.... 

If it arrived on wheels, it's manufactured, has (or had) a  vehicle title, and is financed under special programs for manufactured homes - because of its default risk.If it arrived on a flatbed truck, it was rolled onto the foundation and - for financing purposes - it is the SAME as a stick built home.   

That will let them know that someone knows you are there and will be reaching out to you soon.When I show rental properties to perspective tenants, I look out the front window when the vehicle arrives and text my wife the license plate number or vehicle description.

I was busy setting up my newly arrived family in Canada and just got my WiFi access!

So, it feels great to finally arrive at this point in my life to make it a reality.

The cost of waiting for the next tenant to sign up will be higher towards the end of the year when the holidays arrive and not many are out looking for new places...