I can answer the third question pretty nicely. The aborigines weren't in a position to unite, the social structure isn't a top down chieftain with set lands but family groups associated in larger clans which only really associate with other clans with some shared language or customs. There was a fairly complex trade network for orche, shells, greenstone axe heads and nets but that had some strict rules attached to it, but the trade while facilitating story sharing didn't equal diplomatic ties and organisation. The other and possibly most important factor was connection to country made Aboriginal groups very insular there was no reason to transgress into other people's land because their land belonged to them and they belonged to it.

One side note of interest is the speed of colonisation and technical disadvantage the aborigines had meant there was not much time to organise and with settlers taking a shoot now and shoot some later approach the numbers thinned rapidly.

Trading: The Common Ways Dale Kerin

ABORIGINAL RELIGION AND AUSTRALIAN CULTURE John 0 'Arcy May

Aboriginal Identity and Nation Henry Reynolds

Strictly on a general note, Botany Bay was the initial aim of the First Fleet simply because it was the region where James Cook's first expedition initially spent a lengthy period ashore. Botany Bay was used as anchorage for their stay while botanical samples were gathered by Joseph Banks et al. The voyage north saw features like Port Jackson noted and marked, but not entered and charted.

It was Joseph Banks' reports that promoted the idea of a colony located at Botany Bay, though when the First Fleet arrived in 1788, the bay was deemed unsuitable for large scale maritime activity (too shallow and too open to the elements for a viable port) and Port Jackson was tapped as the closest worthwhile possibility.

1. Why did the settlers land in Sydney?

Because that location was nominated by Captain James Cook and botanist Joseph Banks as a good place to start a colony.

The Dutch explored the west and north-west of the continent in the early 1600s, and weren't pleased with what they saw:

for the greatest part desert, but in some places inhabited by wild, cruel, black savages, by whom some of the crew were murdered, for which reason they could not learn anything of the land or waters as had been desired of them.

Another Dutch captain wrote:

“... that vast regions were for the greater part uncultivated, and certain parts inhabited by savage, cruel black barbarians who slew some of our sailors, so that no information was obtained touching the exact situation of the country and regarding the commodities obtainable and in demand there....” He found the land to be swampy and infertile, forcing them eventually to give up and return to Bantam due to their lack of “... provisions and other necessaries ....”

It wasn’t an attractive land!

Then, in 1770, Captain Cook, with Joseph Banks along, was told to investigate the eastern coast of this southern continent.

Joseph Banks later testified to a British committee investigating the problem of transportation after the colonies in America ceased to be available:

the Place which appeared to him best adapted for such a Purpose, was Botany Bay, on the coast of New Holland, in the Indian Ocean [...]

that the Climate, he apprehended, was similar to that about TOULOUSE, in the south of FRANCE [...]

the Proportion of rich Soil was small in Comparison to the barren, but sufficient to support a very large Number of People [...]

there were no Beasts of Prey, and he did not doubt but that our Oxen and Sheep, if carried there, would thrive and increase; there was great plenty of Fish [...]

The Grass was long and luxuriant, and there were some eatable Vegetables [...] the Country was well supplied with Water; there was Abundance of Timber and Fuel, sufficient for any Number of Buildings

There was a reason that Cook and Banks called it "New South Wales" - they were deliberately comparing what they saw with the verdant landscape of South Wales.

And in answer to OP's second question, the convict Alexander Pearce confessed to killing and eating fellow convicts during two separate escape attempts in Tasmania and there were stories of cannibalism by aboriginals of each other and of gold prospectors during the Queensland gold rush. There aren't any stories about cannibalism by settlers though.

http://espace.library.uq.edu.au/eserv/UQ:242712/Qld_heritage_v1_no7_1967_p25_29.pdf