I'm thinking about both LU and NR in what follows, but mainly NR
(afaik LU only check tickets at the entry/exit points of the system).

When undergoing a ticket check inside the transport system (on the
train or at an interchange), is it enough to have a pre-pay Oyster
card which has been validated at an entry point and is within the
2-hour time limit? Or can you still be PF'd if you're not on a
"reasonable" route? Does the Routeing Guide apply?

This has implications as to whether you can change your mind about
your destination while en route, or start your journey while undecided
as to your destination - which many see as one of the advantages of
Oyster.
The question may become more important as pre-pay is rolled out across
more of NR in London. But at the moment I can't find anything on the
Oyster website about it.

A specific example from the present: Suppose I touch in at Kenton,
take the Bakerloo 1 stop to Harrow & Wealdstone, then get on a
Silverlink County non-stopper to Euston. This looks like it would save
5-10 mins compared to getting an all-stations train from Kenton to
Euston, and therefore might be considered a reasonable route, though
it's not allowed by the NR Routeing Guide. If I encountered a gripper
on the Silverlink train, could I get PF'd? IOW, would I be required to
touch out then in again at H&W if I wanted to use this route? And if
this route is not allowed, would I be "allowed" to use the same route
in the opposite direction, since (I think) the RG doesn't apply to LU?