If the SYD-CNS leg is on the same ticket as the LHR-SIN one then I'd agree - but it seems the SYD-CNS leg will be on a seperate ticket thus I think it's technically a no.

Putting my English language pedant hat on I'd say that the use of 'the' in 'Customers flying in Economy class on the domestic or intra-European segment to connect to or from a oneworld longhaul First or Business Class flight' suggests said segment to be a part of a bigger thing rather than as an addition to something that 'a' would suggest.

But than again - OW rules seem clear in terms of allowing bags to be checked through in such a situation?

At which point I will decleare to be not sure and plead for someone who's actually done this to chime in!

Welcome to FT btw martyn0610!