I think we can predict what TfLs answer to the posed questions will be because of the way the Oyster system works:
1. You don’t need to tap anything, it should just work (subject to maximum times etc.)
2. It is a valid interchange
You’ll never get a different answer to question 2 since there’s no such thing as valid or an invalid interchanges within a station. The Oyster system doesn’t know where you’ve been, it only knows where you’ve tapped – you could (theoretically) travel by balloon, jet ski or teleportation between stations and, as long as you tapped in and out within the permitted journey times, it would likely be counted as a valid journey.
The real question to answer is what happened in the case of the journeys you tried.
The ‘one tap in the middle’ journey is obvious since you terminated your Oyster trip at Greenwich and didn’t start a new one before getting on a train. Without knowing where you started and finished, however, there’s really very little that anyone can offer.
I have one theory (hence the use of the word ‘likely’ in the balloon paragraph) but I suspect it would look silly once I found out where your journey ended so I’ll hang on to it for the moment.