When disabling/enabling tick at runtime, if (TickInterval == 0) then DeltaTime returns as the length of a frame, but if (TickInterval > 0) DeltaTime returns as the length between when tick was disabled and when it was re-enabled
No - same behavior occurs on 4.15.3 (CL 3450819)
DeltaTime returned when Y is released is the time since last time the actor ticked
DeltaTime returns time since last frame
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-45524 in the post.