Code change:
Other fields: As described in canonical-object-fields. <ins> An "OBJECT_PREFER_DATAGRAM" message carries a single object in a datagram or a stream.  There is no explicit length of the payload; it is determined by the length of the datagram or stream.  If this message appears on a stream, it MUST be the only message on a unidirectional stream. An Object received in an "OBJECT_PREFER_DATAGRAM" message has an "Object Forwarding Preference" = "Datagram". To send an Object with "Object Forwarding Preference" = "Datagram", determine the length of the fields and payload, and compare the length with the maximum datagram size of the session.  If the object size is less than or equal maximum datagram size, send the serialized data as a datagram.  Otherwise, open a stream, send the serialized data and terminate the stream.  An implementation SHOULD NOT send an Object with "Object Forwarding Preference" = "Datagram" on a stream if it is possible to send it as a datagram. </ins> 6.3.3. When multiple objects are sent on a stream, the stream begins with a

Related discussion:
text: If the subscriptions were -345, -2, -1, the relay could decide to make two subscriptions upstream - one for -345, the other for -2, because it didn't want to force the smaller relative subscriptions to wait for the larger one. The point is, this is optimization logic for the relay. It does have a choice in what subscriptions it sends upstream based on the timing and diversity of the input subscriptions. Why is the latency of the join affected? Consider a subscriber talking to an edge where the subscriber is 100ms away from the origin. We consider two scenarios: in the first case the subscriber receives a SUBSCRIBEOK immediately after subscribing but then waits 100ms for the audio and video to arrive from the origin. In the second case the the subscriber waits 100ms to receive SUBSCRIBEOK but it is immediately followed by the audio and video. So in both cases the subscriber gets its audio and video after 100ms. The media latency is consistent under either scenario.
If the relay logc is to send only a single subscribe upstream, then you don't need anything to be said in Subscribe OK either. Since when the first object arrives it can compute the live edge by adding 5 to the object being reported and the compute the absolute values for others (in this case for -3 and -1). That would definitely make things simpler for sure.
For cases where a client wants to go back 345 groups in past, i will be very much inclined to do an absolute request after learning the latest known state of the group as an end application OR something catalog can describe more information to make the right absolute request on the first go
It can't do this because the order of items it receives as Streams is not guaranteed. The origin may send the groups in order -5,-4,-3,-3,-1, but they could be received at the relay (or any hop) as -3, -5,-4,-1,-2. That's why you need an explicit notification from the origin as to where the live edge is. Only that plus the group sequence numbers allow the correct sequencing, not the order of arrival.
In this case there was just a single subscribe request with -5 was sent upstream, based on your explanation above. This avoids all the confusion which would happen when there are multiple inflight -ve offsets.
This seems like it will have many race conditions.