Does the easement allow you to continue your journey from St Pancras to London Victoria? I interpreted the easement as valid solely to the stations listed and no further once you travel back south from St Pancras on Thameslink, although I’m happy to be corrected on that one.
Yes, I cannot see why you wouldn’t be allowed to. Honestly I don’t see how the easement comes into it at all. The additional validity permits you to travel the entire length of the Thames core past Farringdon, which cheaper tickets do not permit. There is nothing about not being allow into all south London terminals once on the other side of the Thames.
The fact that barriers may not be able to tell how you arrived is not confirmation of validity. The easement allowing travel south of St Pancras clearly names a range of stations that does not include Victoria so I don't think it can be taken as clear that such a ticket is actually valid.
Granted, whether a barrier accepts a ticket is not the best way to deems its validity. Although I remind you that
Plus High Speed tickets include a 20% surcharge. These tickets allow passengers to use all mainline routes into London, including St Pancras. They are also additionally valid to travel on HS1 and into the Thameslink core with all other terminals still permitted I cannot understand how it can be suggested that more a expensive ticket has less validity.
You are correct the easement does not mention Victoria, although would anyone really question whether it is still valid? Charing Cross and Cannon Street are still allowed on reaching London Bridge. It would be ludicrous to suggest passengers are only permitted to travel to Victoria via Bromley South. Otherwise, we would all opt for significantly cheaper
NOT VALID ON HS1 tickets, as would not benefit from paying more for the Thameslink validity that limits further onward travel options. Presumably they expect people to travel on direct services instead anyway. Time wise there is not much of difference in it.
And would barrier at St Pancras (Southeastern) swallow my ticket?
Potentially, it might do. It’s always safer to show it to the member of staff
You can only use Oyster in PAYG mode on Southeastern High Speed (as Travelcards aren't valid), so the journey won't ever really be 'free'.
Stratford International to St Pancras is charged as a 'special fare', separate from the rest of the system as with the Heathrow and Gatwick Expresses. If you travel from Stratford International to London Bridge, for example, you'll be charged two separate fares, one to St Pancras (SE High Speed) and another from St Pancras (Thameslink).
So there are circumstances where a paper ticket could be cheaper than Oyster.
This is a very interesting point. Someone with a paper ticket would of course be allowed to travel onwards on Thameslink. Although on Oyster, tapping out at St Pancras and then in again downstairs would lead to being charged extra. The system is completely designed to discourage people using Oyster anyway.