But if you do not touch the right reader you could end up on a train with no valid ticket and be accused of fair evasion for no fault of your own. I do not think TFL should be operating out to or setting fares to Reading.This is just a problem of the railway being in a period of transition whereby a new operator has introduced services on the Reading to Paddington route and it has a different fare structure, which some see as highly simplified, charged solely on a point to point basis, but the old fares still exist alongside.
There is a view that government (and railway operator) policy is that the TfL structure will be adopted in the near future on this route but to do so will create some losers as well as winners. There are also practical difficulties about this (Contactless PAYG) being the only fare structure so they have to coexist for a period. A one-off change in December 2019 might not have been acceptable but, to take your point about confusion, it would have been immediate rather than drawn out.
In practice, most regular users learn what is the best value for their journeys and use Contactless or 'paper tickets' depending on which will work best for them. An occasional user might look into what will be best before travelling.
The public is informed about fares on Oyster and Contactless - it is all on TfL's Single Fare Finder.
"NR-NR OSI" means that there is no "out of station interchange" (OSI) between the Network Rail barriers on platforms 10 to 14 and those on platforms 2 to 5. Essentially, that if you use Contactless at Paddington, going through any set of barriers on the mainline station and then another will end the first journey and start a new one. There are very few journeys where someone would travel in and out of Paddington in any case (although I note the point you have raised about there only being 1tph GWR service on the relief line currently).
We need one fares system, my point all along. The railway is now running as a cash cow for foreign operators and investors and not as a service to the public.You touch IN at your origin and OUT at your destination.
When interchanging, you may need to tap out and back in again.
Routeing Guide easements are not applicable to Contactless/Oyster PAYG; it's a separate system.