There is nothing to suggest the older couple were ill or disabled.
If they were, they ought to have booked Assisted Travel. This means a guard carries on your bags, sees you to your seat and generally makes sure you are ok.
I know older women aged 80-90 who travel alone on trains who use AT.
As for reservations being null and void owing to fewer carriages- how do we know if this is true?
There is no excuse for taking a seat not reserved for you, otherwise.
I've seen this a lot on trains in the past and when asked to move, people do. You can see where they reservation starts and ends, so it's okay to sit there until the owner of the reservation boards which might be a few stops up the line, but then you have to move.
Am all for showing it on SM but as you can't see their faces I doubt they will be put in the stocks.