Thanks for clarification. IMO, I think the readStatus should reflect:

1. "success" -> sender requests the read report and the receiver already reads it for sure.

2. "pending" -> sender requests the read report but the receiver doesn't read it yet.

3. "error" -> two possibilities as below

    a. sender requests the read report but the message is 'deleted without being read" or
    b. the message is not successfully delivered (i.e., deliveryStatus = "error").

4. "not-applicable" -> sender doesn't request the read report or it's an inbound message.

So, I think you can just add some words for 3-a in the existing descriptions. Actually, you already mentioned to set it as "error" for "Deleted without being read" when describing the steps. Right?