It's not so straight forward as that, as many people would want to options that include short haul Y seats. Nobody wants to see a J award displayed when it is MEL-SYD(J), SYD-LAX (Y), but most would want to see a J award displayed when it is MEL-SYD(Y), SYD-LAX (J).
Not to mention int PE or F flights with domestic connections - since there are no dom PE or F services, then basically you won't be able to see any results for PE/F going by the aforementioned proposal.
It is not the availability of Business Class seats that is under scrutiny here, it is the process of advising people they are available when actually they are not. Some people, like myself cannot fly long haul in Y seats without risk of further injury. So when I attempt to book J seats only to discover I am actually being asked to pay J seat points for Y seats it is a form of smoke and mirrors that is similar to promising a wheelchair user that a building they want to go to is accessible, only to arrive at the building to discover a flight of stairs and no lift. Definitely not in line with the DDA imo.
Whilst it may be "sneaky", the booking engine clearly states what you will be paying for (eg one sector J, the other Y) and you have to agree to the terms before purchasing. It is technically impossible to not have the opportunity to see those details online.
Re first bold selection: it actually doesn't occur, specifically it does
not advise you that there are seats available in the same class for all sectors in your selected itinerary.
Re second bold selection: again, that is not what the system is asking. In your example it is asking you to pay J points for a
combination of J and Y seats.
Whilst I empathise with your situation, the shortfalls of the booking system do not absolve you from any responsibility in being careful before you purchase flights that clearly outline the terms. Well, they are only clear if you read them. If you choose not to read them, of course they won't be clear :!:
Searching each leg separately in Multi-city view might help with that.
Tried that myself, can't seem to get the multi-city to do that for an ASA. Classic - yes, ASA - no. Would love to see how you are doing it