19c, which allows tickets that would ordinarily be invalid to be used as part of a valid combination, has got nothing to do with this, though. It is an exception to the rule.
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19(a), 19(b) and 19(c) are all, to use All Line Rover's phrase, "exceptions to the rule" (that combinations of tickets are not valid for a single journey except as permitted by the contract), and each are of equal validity. Both 19(b) and 19(c) allow tickets that would ordinarily be invalid if used for only part of the journey to nonetheless be validly used as part of a valid combination.
Just as a London- Milton Keynes SDR and Milton Keynes- Coventry season would be valid to travel on a Coventry-bound train not stopping at Milton Keynes at all by virtue of 19(c), so a combination of 'ordinary' tickets split at Milton Keynes on a 'MKC pick up only' train is valid courtesy of 19(b) alone.
I interpret the sentence in Condition 19 about the entire journey needing to be covered as meaning:
1) that there must be no gaps in the tickets (e.g. London- Bletchley and Milton Keynes- Coventry tickets would obviously not qualify for any form of condition 19 split due to the gap in validity between Bletchley and Milton Keynes), and;
2) that any tickets held as part of a combination must be otherwise valid; for example, a London- Milton Keynes CDR would not be valid on a weekday train departing Euston between 1645 and 1900 inclusive, regardless of whether the train stopped at Milton Keynes.
I don't believe that statements regarding validity by ATOC or by the TOC concerned are necessarily definitive, (especially given the obvious financial incentive in claiming that the tickets are not valid in order to extract more money from the passenger).
The wording of the contract(s) entered into determines the trains the passenger is entitled to travel on, and the NRCoC is incorporated into, and forms part of, the contract between TOC and passenger.
Having said that, a passenger may clearly encounter significant practical difficulties in using valid tickets if rail staff mistakenly believe they are invalid.