I'm pretty sure I do understand your issues. There is an imperative to stick to the plan and that is reflected by airline company management but not always the preference of the pilot of the moment. Some want to shortcut the STAR others want VSA others want different approaches, some want vectors, some don't, none want speed control and of course every pilot is in the sky on their own and wants to be numero uno.
I agree that the STAR should conclude with the advertised approach but what is the difference between a final 15nm straight-in segment being flown visually or ILS or other (apart from your system set-up of course). A different straight in 15 nm final approach would be technically irrelevant to ATC unless the expected airspeed paramaters are altered significantly (and this variance should be a consideration by ATC regardless). If the plate says NDB or ILS I do not see the issue with ATC issuing an ATC instruction permitting any other approach (or cancelling STAR and permitting another approach). There is flexibility to give you what you want most of the time. Yes the STAR could list every possible approach to get you on the ground from a 15nm final point. Also a visual component is an option of refinement of design and/or an option for flight on the last 15nm final segment. No problems there.
So is your issue not having a heads up on what approach you will get or the fact that the words are not on the plate?
On slot times ...... airspace and traffic management always has a dynamic that may include weather, emergencies, unknown events etc and therefore some tactical air traffic control must at times be implemented (and expected/appreciated). The idea is to provide slot times for RPT jets and fit other inbound traffic and departures within those times.
Bottom line is that ATC do not know everything about all things as RPT jet pilots also do not. We need your input to adapt and impove and there is no unwillingness by ATC to do so. ATC attempt to provide the best and safest solution to the whole breadth of aviation operations at Darwin NOT JUST RPT. and often that is a complex and demanding task. Darwin is well known for its ATC challenges so please do not underestimate the ATC efforts to provide the best service they can. I suggest you transfer your thoughts from the PPRUNE page into a precise solution or suggestion and send that in writing to the execs at Darwin. They are good people and professionals and will consider this in relation to effect upon other operations and compliance. Some of these issues are not the domain of Darwin ATC and should be directed to Airsevices Australia. PM me if you need further assistance.