Here is one that I couldn't figure out.
I know that when 2 fare components are combined, the most restrictive rules apply. But is it the same case for the stopovers section?

So if the first component allows stopovers in SIN/KUL/CMB, the second allows stopovers only in DXB, can these 2 be combined in the first place to form an itinerary like this: MEL-DXB/DXB-AAA-DXB-SIN/SIN-MEL (as an example only)?

I tried to price similar itineraries (not these ones) on EK website, but it seems that I could only combine these 2 fares if no stopovers are selected, and the system fails to price the itinerary if both stopovers (one on the outbound in SIN, one on the inbound in DXB) are selected. If only one stopover is selected, the system combines the fare components and prices them properly but then at the booking stage the payment process fails (the credit card doesn't get charged).

I tried to change a current booking (booked with a stopover in SIN without a problem, but am getting the same issue at the pricing page.

Thanks

I forgot to add, that trying that above with different cabins (so J outbound, Y inbound) seem to price fine but didn't really try to book it because the Y component come with a pricey penalty. So don't know whether or not it will still fail at the payment stage.

It looks to me like you are trying an 'end-on-end' combination of two return trip fare components i.e. a MEL-DXB fare and a DXB-AAA fare. In my experience EK and most other carriers do not allow end-on-end combinations on discounted fares, as it makes it harder for the airline to control their pricing.

Have you tried pricing the itinerary on matrix.itasoftware.com ? It can often offer clues as to how journeys are being priced, and you can access fare rules there also.

With that said, EK's online booking system is not perfect - stopovers are generally fine, as are open-jaws and side trips, but I've never tried an end-on-end.

I am not trying an end-on-end combination. The fare components I am referring to are the same origin/destination (one is ART, the second is FRT).
matrix ita fails to pull these fares and instead pulls IATA YY fares.
It's a similar story when using an amadeuspower engine.
If no stopovers, it pulls FRT for both. If with stopovers in SIN it pulls the EK YRT that has unlimited stopovers.

I am guessing this is to do with the 'most restrictive' conditions and thus sees the combination of ART/FRT as invalid.

If you have 2 componants such as ART plus FRT iirc the restrictions of ART will apply to ART and the FTR restrictions to FRT

If ART allows stopover in DXB only then you can stop in DXB only on the A direction
If FRT allows unlimited stopovers then you can stop wherever u like in the F direction

I have also found that the online booking tool has issue handling stopover rules and you may find that phoning and speaking to an agent and explaining the situation can deal with issue

I also noticed that when adding 1 leg (such as CBR-MEL or MEL-CBR) the intended fares get pulled as well as QF's LFLEX fare, but again did not try and book because this QF fare shows as non-refundable.

if starting or ending in CBR then there may well be EK fares from/to CBR that can be used rather than adding a separate fare. Again these are things that I have found the website to have issues with

When wanting to book a CBR-DJ-BNE-DXB-LGW flight, I have had to phone EK to book it since the online tool just wouldn't price properly even though EK has a fare

I would really recommend just calling reservations and they should be able to read fare rules and tell you for certain what the fare can do and book it.. plus if there is issues with auto-pricing correctly they can pass it to the fares department to deal with