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).

Well they have the "random" ticket inspections of course, but basically that
sounds right.

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?

Interesting one. At the moment it seems that the NR routes that Pre-Pay is
valid on are very specific so there shouldn't be an issue with a route being
unreasonable. On the tube I guess there can be considered to be
unreasonable routes on Pre-Pay. If for example you touch-in in zone 1 and
are found to in zone 4 heading towards zone 3 without having touched-out in
zone 4 in between it is going to look dodgy though you could of course have
just missed your stop and be intending to touch out in a zone 4 station.

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.

Presumably if Pre-Pay is rolled out across more of London it will be on a
zonal basis so the route will become less critical.

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?

The Oystercard website doesn't really make it clear, but I've always assumed
that you are expected to revalidate when switching between tube and NR, or
vice-versa. So you should touch-on at Harrow & Wealdstone.

I've also never really been clear about how smart the validators are. To
switch the scenario to stations I am familiar with, if one switches between
tube and Thameslink at Farringdon, is one expected to touch-out on the tube
platform and in on the NR one (or vice-versa) or is just one validation on
either sufficient for the system to sort it all out. Similarly if switching
between tube and Tramlink at Wimbledon, is validating on the Tramlink
platform sufficient for the system to register a touch-out from the tube
system?

I do find the 'Pre-Pay users touch-in here' type notices hopelessly
insufficient in many cases. Why not have a notice clarifying exactly who is
expected to touch-in?