Ambiguities on pickup protocol

On the pickup protocol v3.0

Why is the 'thid' only shown in the example for `delivery` (Message Delivery type)?
Does that mean that is the only place not optional?

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Is not clear if `messages-received` message can exist without the 'thid' field.

Since the "`message_id_list is a list of ids of each message received. The id of each message is present in the attachment descriptor of each attached message of a delivery message.`" and "`The id is an opaque value, and the recipient should not infer anything from the value.`"

The recipient can not assume the 'id' is unique. The way that the protocols are described, the 'id' will only make sense in the reply.

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IMO 'thid' should be optional on all message types of this protocol. Which means the execution of the protocol can start anywhere.
But for that, the recipient needs to assume the attachment's 'id' is unique to the mediator.

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In the Delivery Request, we have:
`If no messages are available to be sent, a status message MUST be sent immediately.`

IMO we should just remove this line.
Why are we changing the behavior in this condition (assuming that by `immediately` you mean `instead`)
Sending a `status` doesn't bring any extra value.
For the recipient is the same have a `status` saying that you have zero messages or have a `delivery` with zero messages. But for the developer implementing a state machine is more complicated.