I am not sure about message table not having delivery.But, I can help with defining the variable in delivery and using it later in your workflow. To define a variable in delivery activity, Right click on delivery and click open. It will open a generic workflow activity form instead of delivery form....

Hi, Below might help 1. to retrieve internal name for any workflow activity you need to use activity.name,this might work for you2.  For AB testing, you can find more detail on below link. You can use delivery code for AB testinghttps://docs.campaign.adobe.com/doc/AC6.1/en/WKF_Use_cases_A-B_testing....