Apparently the service between Stratford International and St Pancras International isn't valid on a travelcard, despite being within the area. I think it's quite disgraceful, and I bet hardly anyone uses the trains for this journey.
Correct on all three counts! You can't use an in-boundary Travelcard for a BZ extension ticket either. I believe the only Travelcards valid on HS1 services at all are those issued by South Eastern from stations outside the zones and routed via HS1.
I'm surprised that Stratford wasn't made a pick-up/set-down only station for trains from/to London St Pancras; having allowed journeys you would have expected that Travelcards would be valid so that South Eastern would (presumably) get a slightly increased amount from the Travelcard revenue pot.
If what I understand about the validity of Travelcards is correct, it would lead to the curious situation where someone with a Romford to London Terminals monthly season could use HS1 from Stratford International to St Pancras (using the 3 mile rule - as the ticket is routed Any Permitted) whereas someone with a Romford to Zones 1-6 monthly Travelcard couldn't.
Is my understanding correct?