Rayners Lane terminator

There are no scheduled Met trains which reverse at Rayners Lane, so it would have been a late running train or one which happened to be in the wrong timetabled path after an earlier operating issue.

A signal failure at Aldgate East was disrupting H'smith & City services until 19:15 between Moorgate and Barking. As is the classic case with the SSR, this gave the Met some grief and inevitably led to services being cut short. We can only assume that Harrow-On-The-Hill had a limitied reversing capacity at the time so Rayners Lane was chosen to squeeze an extra service in to reverse there.

Are announcements ever made on Baker St terminators from the suburbs that it's easier to change at Finchley Road for destinations beyond Baker St than it is at the latter--given a potentially long walk/stairs to deal with?

You just said yourself, "potentially".

If a train were to terminate on Platform 4 passengers for through Aldgate services have cross platform interchange with Platform 3 but only if the next service to Aldgate is a Metropolitan service which in many instances isn't the case.

If the next service via Aldgate was a Circle line service and a Met service terminated at Platform 1 (or 2 for that matter) passengers need simply walk via the large gates (Platform 2) the or smaller interchange corridor (Platform 1) leading down to the Eastbound Circle/H&C platform without stairs.

Even though some drivers are proactively aware (from the WTT) of the platform that has been allocated at the terminal location and station staff have copies of the Working Timetable (WTT) to hand a clause in it refers to routeing of trains that, "Signal Operators can, however, vary routeing and platform working in the best interests of the service."

Effectively the advance information known at Finchley Road is too conditional to make any meaningful announcements to passengers which is why normally they aren't made on services terminating at Baker Street.

I don't think drivers are aware of which platform they are going into, and I don't think they care, as long as the route provided is sensible it will be accepted without thought.  Going into Baker street a driver has no interest as to whether the train is going to be routed into platform 1 or 4 and will accept either, questions will only be asked if 2 or 3 is routed.

Indeed, official Metropolitan Line drivers duty books do not give platform numbers for trains at Baker Street.

The S stock CIS does have Rayners Lane and Ruislip as settable destinations for NB trips. It also has Ickenham and Rayners Lane as SB destinations. I'd be surprised if Ickenham gets much use but it covers reversing via the crossover at Ruislip siding and going back to Uxbridge. Rayners SB then involves a double shunt via the siding, so really for service purturbations etc.

Well it might be if there was a signalled south-north move over either trailing crossover, but alas not.

In fact, according to the diagram in @martin12's post, the trailing point in both cases is a spring point. I'm not certain that's still the case, but it certainly was the last time the area was resignalled, so the points would have to be secured for any move over them in any circumstances.