@vombatis, love the logic and graphic.
Did you consider the tilt of the earths axis?
Warning, science content follows. Read on at risk of confusion.
The earth is not round, but a bit squat.
As the earths surface is curved no 2D map (projection) can exactly match the curved surface. The earths axis is tilted big time.
The earth does not rotate 360 degrees in one day (24hours).
The assumption in ‘C’ might only be correct on two days of the year around the dates of the spring and autumn equinox.
It may assist to have two more example graphics, one for each equinox to illustrate the effects.
In AEST for all times
On 22 June 2019 sunrise - sunset
Cairns 6:45 am - 5:52 pm
Brisbane 6:37 am - 5:01 pm
Melbourne 7:35 am - 5:08 pm
On 20 Dec 2019 sunrise - sunset
Cairns 5:39am - 6:48 pm
Brisbane 4:48 am - 6:41 pm
Melbourne 4:53 am - 7:41 pm
In mid winter the tilt of the earths axis approx 23.5 degrees aligns roughly with the east coast of Qld. Which is why the sun in Winter in Bris is up at 6:37am, 58 minutes before Melbourne.
In mid summer the tilt of the earths axis is approx 23.5 degrees the other way and roughly aligns with the SE coast of NSW. Which is why in summer sunrise in Brisbane and Melbourne are only 5 minutes different.
Noted in summer the sun sets a whole hour earlier in Brisbane. As you point out in ‘A’ the further south you are the longer the day in summer. The situation reverses in winter with Brisbane having an hour more sunlight than Melbourne.
Separate to any discussion of the need for daylight saving in any state, note that for Qld in summer
Sunrise in the north EG Cairns is 51 minutes later than Brisbane, and in Cairns the day 44mins shorter. Compared to the far south of Australia Cairns is short up to 2 hours day light in summer!
The current 150 degrees east line of longitude as the basis for AEST time is a compromise.
Greater Brisbane might swing either way with DS? In summer the rest of the state lags Brisbane by more than an hour in daylight terms. It was probably the key factor in DS being rejected in Qld twice previously.
I doubt Daylight Saving is a friend. It’s complex with diverse view points and competing needs.
Note edits made to correct grammar only.