I regularly take trains from Leeds to London, after taking a local service into Leeds. Typically connections from Bramley, New Pudsey, Apperley Bridge or Kirkstall Forge.
There has never been an issue taking an earlier connecting service.
Good to hear the Guards take a pragmatic approach.
However, is this official Northern policy (it is GTR's), and can you guarantee revenue protection officers will take the same pragmatic approach?
Northern revenue protection officers are known for having people among their ranks who are the opposite of pragmatic; many of them are arguably not fit for the job they do (but then I have different expectations of employees than senior staff at Northern do), and certainly wouldn't be suitable to be employed as Guards.
If anyone can identify they Northern have an official policy, then great, but until then I would not take an earlier service without permission from the Guard.
Seats can't be reserved on the Northern local connecting services.
This isn't relevant; Northern, Chiltern, Greater Anglia, WMT, GTR and others offer Advance fares that are not tied to a specific seat.
My reading of the original post is that they do hold a reservation coupon, stating a specific service, from APY to LDS, but with no specific seat, which is absolutely normal for all Advance tickets on Northern services, and this applies regardless of who sets the fare.
In contrast, Northern certainly did and - without checking - probably still do operate some services that have no reservations whatsoever, in which case any "appropriate" connecting service can be used into the booked service, but that doesn't appear to be the case here, based on my understanding of the wording of the opening post.