Hmm. That's disappointing.
I'm slightly surprised at your disappointment, not only because, as a diligent passenger you will no doubt have read the conditions of the ticket before you bought it and were therefore aware of the break of journey restriction - but also because this is the rail industry and no-one does faff and needless complexity as well as the rail industry!
The barriers at Kings X do accept the tickets, as I've made the journey a few times in the past.
They accept that on an Advance? That would surprise me a little, but if that's the case then it's obviously one less potential area for trouble.
Playing Devil's advocate, what grounds would they have to issue an excess fare or penalty? Is there a maximum allowable connection time?
The excess fare is payable under the terms of the National Rail Conditions of Travel, to which one is required to agree in order to buy the ticket, as outlined by cuccir. I don't think a Penalty Fare would be payable, for a number of complex reasons. There is no defined maximum allowable connection time anywhere in any conditions or industry data.
What if I decided to walk from Victoria and stopped for a pie and pint on the way? Or got lost?
The previous Conditions of Carriage, which were retired some years ago, held that you were not breaking your journey merely by using station facilities. Accordingly, if you decided you fancied a pint or two at the Parcel Yard at King's Cross, under those Conditions you were clearly entitled to do so with your ticket. Now that the definition has been removed it is less clear, but I don't see why the previous definition should be totally disregarded, seeing as it has not specifically been overridden by something new.
I would say it simply comes down to a 'reasonableness' test. If you are eating something along your way because you are peckish, you are still making your journey, even if that entails missing a train. But if you decide to stop off at an art gallery then clearly you are no longer undertaking your journey.
In what way would a train several hours after my arrival at Victoria not be "appropriate"?
There is no fixed definition of "appropriate", but the phrase "connecting trains" clearly implies that it is supposed to form one and the same journey. As stated above, if you start undertaking a task that has no relation to your journey from Maidstone to Attleborough - e.g. visiting a museum - and therefore missed several trains, I don't think that the train you ended up catching could reasonably be classed as "appropriate".
What if I took a print from a journey planner showing the actual train I get from Kings X and displaying the exact same price as I've paid for my ticket?
The price of another ticket that you don't hold is irrelevant to the validity of your ticket. Next time you can of course use a journey planner that lets you stretch out the journey by adding 'unnecessary' connection time, e.g. TrainSplit.