What counts as a day of 'travel in your own country'?

I'm planning to use my Interrail mobile pass (15 days in 2 months)
(1) to travel from Stuttgart in Germany (where we live) to England (Stuttgart-Strasbourg-Paris-London) together with my 10 year old son,
leave my son there for a few weeks with his mum
(2) come back to Germany for work (I'll buy an additional ticket from the German border to Stuttgart)
(3) return to England to meet my son (I'll buy an additional ticket from Stuttgart to the German border)
travel around England with my son using the Interrail pass
(4)before my son and myself return to Germany, again using the Interrail pass.

(1) & (4), the journeys from Germany to England and back using the Interrail pass seem straightforward.
(2) my coming back to Germany alone, with my own ticket for the German travel in between seems straightforward too,
because (as I understand it) I can use the Interrail pass to travel from London to Paris to Strasbourg (in France),
and then just buy a ticket for the short journey from Strasbourg to Stuttgart
(although by mistake I've made a Rail Europe reservation for the whole journey from Paris to Stuttgart
- is this likely to be ok,
or should I change this to a reservation from Paris to Strasbourg,
so that it's clear which part of the journey is Interrail
and which part I'm paying for myself?)

However, (3), when I travel, at my own cost, from Germany to England again,
things looks rather more complicated,
because on Monday 26.8.
the Stuttgart to Paris trains (TGV 9508, for example) won't be travelling via Strasbourg
(there's planned engineering work on the line)
but instead via Saarbrücken in Germany
(which Deutsche Bahn list as a Grenzbahnhof, or border station)
and Forbach, just over the border in France.

The problem is that there doesn't seem to be any way to make a reservation from Forbach to Paris,
as that station doesn't appear to be recognised either by the Rail Europe or SNCB system on that route (probably because it usually isn't on that route).
There aren't any other scheduled stops between Forbach and Paris on TGV 9508, so I can't simply buy a ticket to the next station.

Is there any way around this?
If I used my Interrail ticket  for the journey from Saarbrücken to Paris, would this trigger a third home country travel day and take me over the limit?
or is Saarbrücken a station on the German/French border that's exempt and that doesn't count as home travel in Germany, by any chance?

One more question - if there IS a way to reserve from Forbach
and I travel on a later train that doesn't actually stop there
then would this be allowed?
or is it like TrainSplit in the UK
when the train has to physically stop in the station wherever the ticket is split?

The same question arises if  I travel from Stuttgart to England via Brussels rather than via Paris:
if I paid for my own travel, say on ICE 16 within Germany,
could I avoid triggering a 'home travel day' by buying a ticket as far as Aachen (which is another Grenzbahnhof, or border station, albeit one within Germany),
and then travel onwards to Brussels with Interrail?
or would I need to cover my own journey over the Belgian border to Liège-Guillemins (which isn't much further but which is a considerably more expensive ticket)?
or could I buy a ticket to somewhere right on the German/Belgian border even if the train doesn't actually stop there?

All thoughts, suggestions and ideas gratefully received!