Hertford North and WGC for Oyster is correct. Many take the view that the Great Northern inner services will get taken over by TfL, if this were to happen then Oyster would ned to be extended to Stevenage, Watton at Stone, Knebworth and Welwyn North as there's no way TfL would operate a service on which Oyster could not be used.
I disagree on part of your post ie the fact that you have stated that were TfL was to take over the GN Inners that Oyster would need to be extended to Stevenage, Watton At Stone, Welwyn North and Knebworth as there is no way TfL would operate a service that Oyster cannot be used on and I will explain below...
Firstly I don't think there are any plans to extend the existing Welwyn Garden City stopping services to Stevenage especially when you consider the bottleneck that is Digswell Viaduct which restricts the number of available paths but also as WGC is the terminus of these services ie you can't go any further with the stopping services it makes sense that it is the furthest you can go with Oyster.
Secondly the Hertford Loop presents something of a issue or not, the issue being that if TfL were to take over the stopping services via Hertford then either they become responsible for the entire service that terminates at Stevenage or they do what First Capital Connect and South Eastern did with the Sevenoaks services which had at the time SE being responsible for the service south of Blackfriars and FCC being responsible for the service north of Blackfriars which means that TfL is responsible for the service from Hertford to London and GN is responsible for the service onto Stevenage.
Thirdly I don't see why Watton At Stone or Stevenage needs to have Oyster acceptance as even if TfL took over full control of the Stevenage to London via Hertford services, you would just need to get a ticket from either of these stations to the boundary/first station in the London Zones ie a Stevenage to Hertford Stations ticket and then a travelcard.
Fourthly you have existing services serving London which Oyster is only valid on part of the journey ie the Waterloo to Windsor and Eton Riverside so I can't see why TfL if they operated services between London and Stevenage via Hertford that they would have issues with Oyster.