OK. If I can get a CDR from Stratford to Hackney Central and back, can
I not get a CDR from Stratford to Canning Town and back because PAYG
can be used on the latter?

The simplest way to solve this is to ask the National Rail website.
You can get a CDR for the latter journey - £2.10 but only on
Silverlink.

Standard day return is £3.



So this proves that a CDR and PAYG *can* coexist. :-)

£2.10 CDR on Silverlink, Stratford to Canning Town and back.
or PAYG.

If both "fares" can coexist on the same line, then I don't see why the
eventual introduction of PAYG on all London NR stations would necessarily
mean that CDR would *HAVE* to disappear on these lines.