does the following itineraries eligible for F Lounge or J Lounge access at HKG / Transit Point?
1. HKG-SIN (CX J) -> SIN-SYD (BA F)
2. HKG-NRT (CX J) -> NRT-LAX (JL F) -> LAX-YVR (AA F)
3. HKG-NRT (CX J) -> NRT-SYD (JL F)

You should be able to use OW F lounges all the way for all itineraries above.

However, for LAX-YVR, Admirals Club does not have a F section and you will need your previous boarding pass for the access. And the access is available to you only if you transit same-day of your arrival.

None of the above will entitle you to F lounge access in HKG as your departure flight is in business class.

At the connecting airports (SIN, NRT) you will be entitled to use the relevant F lounges as you are departing in F. In LAX if you can get access to the F lounge you are entitled to use it, same for the AA flagship lounge. Otherwise you can use the AAdmirals club.

OW lounge access rules for class of service are for the connecting point between oneworld flights, they don't determine the airport lounge at the point of origin. (Individual airlines may have more generous lounge access at the port of origin especially when travelling on mixed itineraries, but as you are mixing your airlines, CX will determine lounge access in HKG, and that's business class.)

Just use the LAX TBIT-T4 airside connector to enter TBIT which will give you access to the QF Flounge on a over 5 hour inbound international F itinerary.

There is no F lounge at SIN, but you will be able to use the BA Concorde Bar FWIW.

It is correct that AC does not have a F section, but there is FS in LAX T4 for OWE/F passengers.