Demonstrating the problem via a video and an explanation that the problem occurs when pointing down your driveway should be more than ample in proving a problem exists. Then it's up to the dealer service department to localize the problem to a component to resolve the cause. The dealer service staff are supposed to be the experts after all.
But as the problem vehicle is a 2015 model, that you recently purchased, it may be that the manufactures warranty has expired since it is around 3 years old. So check the warranty status with Holden Customer Care to be sure. Regardless of the fact the manufacturers warranty may be expired, you still have a statutory warranty defined under Australian Consumer Law to fall back on. You also have a 3 month or 5,000 km statutory warranty on a used car to fall back on (though i'm not a fan of this warranty if the seller is not a Holden Dealer). My preference has always been ACL statutory warranty when the claim against a manufacturers warranty is denied or the manufactrurers warranty has expired.
It may be that the dealer, though it will most probably be you who, needs to discuss this issue with Holden to clarify what if anything Holden will cover under Australian Consumer Law or good will if their manufacturers warranty is expired.
Simply put, you should be covered under ACL
So don't delay, put it to the dealer that it is unacceptable that such a young vehicle should have a starting issues when the car is on an slope. Be clear and tell them that you believe this issue constitutes a minor fault under Australian Consumer Law. Tell them you want it corrected so that you can rely on the vehicle to start even with low fuel in the tank (as any normal person would expect such to be the case).
If this issue can not be rectified as it's considered 'normal for this vehicle', tell them that you then consider the vehicle is 'unfit for purpose' under ACL as you can't rely on it starting when you need to drive to work in the morning. Then tell them that since it's unfit for purpose, you want a full purchase price refund as your chosen legally entitled remedy.
Sit back, wait for what you have said to the dealer and Holden to be digested and then it's likely all will be fixed at no cost to you
Finally, dealer diagnostics should never be a slap-shot process of replacing parts until they finally get to the faulty part, while billing you for all the parts and their lack of skill. That's just an old con job approach to fleece you of your hard earned. And note that vehicle repairs are also covered by Australian Consumer Law so service or repairs must be carried out in an appropriate standard, which slap-shot parts replacement is not.
