To suggest that two major carrier, e.g. SQ and QF don't have an interline agreement, as this reply from SQ suggests, is nonsence.
I don't question the part about not being in partnership, which is true, but I think the word partnership was purposly chosen so as to skirt away from the point of interlining. There are many levels to interlining and the agreements will vary between carriers, the logic being that more the startegic their partnership is, the more comprehensive the interline agreement between them.
But most IATA member carriers will interlline baggage from one to another, even if they are not commercial or stretegic partners. And the journey doesn't have to be on the same ticket for the baggage to be through-checked to final destination, although proof of CONFIRMED onward/connecting travel, i.e. the onward ticket will be required at check in to secure baggage through check.
As to baggage allowance (when interlining), this is very much regulated by the IATA convention, to which both, SQ and QF subscribe and can therefore collect revenue on each other's behalf. However, all this is a lot of work for a check in person to process for no reward or incentive, and when this other carrier is from the opposition camp on the top of it, why would they bother? So, how to avoid it? Exactly the wat SQ responded so as to throw you off track and discourage you from even attempting.
The good old days when these things were done between carriers as a matter of courtesy (knowing that what goes around comes around) are long gone, thanks to the arrival of various alliances. In the old days of travel, what is today possible within one alliance was back then possible within the whole community of IATA member airlines. Not so sure that alliances have brought all that much benefit to the traveller, but they certainly have to the airlines.
Anyway, that's a whole new discussion ... To answer your specific question, it's hard to predict how the first check in agent will behave and either accept to through check or not to final destination - I belive they should but power is ultimately in their hands and you won't be able to force them to do it.
Either way, with such long transit time, even if you need to collect and re-ckeck your bags, you should be able to do it, assuming no significant delays to your first flight. Do check immigration regulations for transit country though as what would have bee transit in the event of through checked baggage, to collect the baggage mid journey, you will need to enter this transit country.
As for the the baggage allowance in the case your bags get through checked, on the second leg you are not entitled to the first carier's allowance and shoudl not expect to get it. If the QF check in agent at the original departure point were to not enforce the second carrier's (SQ's) ticketed allowance (or alternatively collect the excess baggage charges on behalf of SQ), then SQ would most likley bill QF for not collecting, hence why it is easier for airlines to just simply avoid this predicament by claiming they can't interline bags. Hope this is helful to you ...
