I linked the T&Cs of the pass in the last thread you posted. Here’ the relevant paragraph.

There can be no definitive answer, because it’s somewhat open to interpretation. But to me, taking a journey to London several days after you return home is not ‘travel from the border or an airport or port back to any location in the country of residence’. 

They don’t specify that it has to be the same day, or perhaps the day after, you arrive by plane. But the clear intention is that the inbound journey is to return home - not for another jaunt in the UK.

Whether you will encounter any problems making that journey is a different question. 

An Interrail Global Pass and an Interrail France Germany Pass, may only be used for two specific trips in the country of residence of the traveller (provided this country is an Interrail participating country). These two trips are referred to as the outbound and inbound journey.

• The outbound journey can be used to travel from any location in the country of residence to the border or an airport or port.

• The inbound journey can be used to travel from the border or an airport or port back to any location in the country of residence.

There should be no problem for you to use your inbound travel day for a travel in the UK. But maybe it is better to use the travel day from London to Manchester than the other way around. In that case it is easier to claim that it is an inbound journey from London to your hometown. 

Yes, you’re right. That’s what I was thinking. Use it the other way round. I have only used one inbound/outbound day with our trip, with flying home from Spain. Many thanks. 

I really wouldn’t mislead the train staff about what your journey is. Travelling without a valid ticket is one thing, and might risk a penalty fare. Lying about it, and claiming to have flown into London - that would be a lot more serious!

@Alby1979, you seem to want to only hear the answers that will let you travel. That’s understandable. You probably won’t have a problem in practice - they’ve never given more than a glance at my pass. But that’s not the same thing as a definitive answer saying that this is a valid journey. It’s clearly outside the spirit of the rules. And when train companies get wind of this sort of thing happening, it just makes them tighten everything up.

Please be warned that if a ticket inspector does decide to ask you about your international journey, it’s unlikely you have leg to stand on.

This is a contentious debate but I would see no reason for a ticket inspector to query your travel plans, and even if they did your pass is showing a valid ticket for the listed journeys, even when that travel day is separated from any cross border journey.

You have bought a pass that entitles you to 4 travel days within a month AND entitles you to a maximum of 2 travel days where your country of residence is included.

The wording of the T&Cs and the management of your use of those 2 days within the app appear contradictory, but IMHO the Interrail guidance seems to be taken from the paper pass, where management was immediately visible to inspectors in your paper record.

The app simply detects when you travel on any train with a scheduled stop in your country of residence and adds it to the 2 available COR days. Once you have used both days it will not create any valid journeys with a stop in your COR.

I have used a pass for two inbound journeys with no problem, and there is no requirement to use a train to leave or enter your COR.
In over 30 journeys using a Global Pass I have never been asked on inspection for my reason for travelling or future ongoing journeys, nor would I expect them to unless it was on a late running train.

The fact there is not a definitive answer is quite frustrating. I certainly don’t want to do anything I shouldn’t be doing - and wouldn’t - I wouldn’t risk a fine or lie in order to use the final day of my pass. 
But as I can add the suggested travel plans to the pass, I assume it is within the rules and Ts&Cs or the app wouldn’t allow it. 
Of course, I would love to use the final day of the pass - we’ve had a wonderful experience with it so far and would definitely use Interrail again. 
Thanks all for your help and thoughts. 
 

I would agree with @Yorkie on this, you have paid for 4 days including the 2 days in UK you should use it if you can.

It is well out of the scope for an inspector to demand proof of other travel arrangements.

It is not stated anywhere in the conditions that the pass use has to be a direct journey from an entry port to your home or that it must happen on the same day.

I cannot see why you should receive a fine or be made to buy a retail ticket. As far as the inspector is concerned as long as his train is listed on the travel day you have a valid ticket to travel.

If the inspector had any concern about the COR rule, highly unlikely he is even aware of it as he does not know your COR, he should simply take the details and pass that on to Interrail for their consideration on modifying the app. 

This issue has been raised many times and each time the IR staff has seen them and, as far as I know, never contradicted the interpretations, allowing the app to do the control. This is not a surprise to me as you have not used the pass in any way that costs them any more than using it for 4 days outside your COR.

I believe that in the early days of Interrail you returned your completed paper pass and then they were analysed to calculate operator payouts. Now I believe they use a totally different methodology and there is no use of your pass usage. So there is no change in payments to participating operators based on real pass usage. I suspect it is like bus passes in the UK - bus operators/councils gather statistical usage data to determine their payback.

Eurail BV gathers statistical data indeed, from the mobile passes, which is about 90% of passes, and therefore accurate enough to determine which operator gets what.