Let's take a look at these theories:

A) The Kokoda Trail campaign stopped Japan from invading Australia, a last stand by ANZAC forces.

The entire point of the Kokoda Trail campaign boiled down to the Japanese command wanting to neutralize Port Moresby as a base for Allied forces to strike the West Indies. Essentially, a bright army officer had the brilliant idea of landing a single Japanese division, just like in Malaysia, and having them march through New Guinea to Port Moresby, just like they did to Singapore. As such, they essentially gave them a month of supplies, shipped them down to the Western end of New Guinea, and said good luck.

Unfortunately, if you've actually seen a map of Papua New Guinea, you'd realize that the place is not only a good 3x longer from tip to tip than Malaysia, but is also far less developed and far more prone to things like malaria. Not to mention the Kodoka track itself is essentially a small, beaten path, unfit for anything but walking, making supply logistics next to impossible.

Against this emaciated malarial division, the "last stand" of the Australians was only outnumbered by a ratio of 1:2-namely, the Australians had only about 30,000 troops in Port Moresby to deal with the Japanese 15,000. To be fair, malaria was rampaging through the ANZAC troops as well. But they were defending, and had a port open for logistics and supply to the troops. So to consider it a "last stand" seems to be a bit of a fantasy.

Now, did the campaign stop the Japanese from invading Australia? No, for three reasons. One, Japan had only about ten divisions for the entire South Pacific. Of these, two-three were in the DEI, 3 were in Burma, one-to-two were in the Philippines, and I believe one was in the Solomon Islands (specifically, Rabaul). This left a whopping two divisions in reserve for deployment. Simply put, there was no way 30,000 troops would EVER be able to conquer Australia. And one of those divisions was busy dying to malaria in New Guinea.

So it clearly wasn't the Kokoda trail campaign that stopped Japan from invading Australia.

B) What about Guadalcanal?

Firstly, the battle of Guadalcanal was fought, not to stop a Japanese offensive, but to secure an air base for Allied land-based aircraft for a campaign to secure the Solomons as a base for Allied forces to attack deeper into Japanese territory. If anything, the Japanese were using Guadalcanal (at least, they were intending to) as a defensive air base against Allied assault. It is arguably feasible to use it as a base for aircraft to raid Australia, but Japan was already perfectly capable of doing so with their carrier strike force (the Kido Butai), as given by their raid on Darwin in 1942. Even diminished by Midway, the carriers could still pose a threat.

Secondly, the deployment of the Japanese Navy at Guadalcanal suggests a defensive posture. If the Japanese were truly intending to invade Australia, the main Japanese fleet would have logically been deployed at Rabaul, However, it was deployed at Truk, which is much further north. The reason it was there was that it was roughly equidistant to all parts of the empire in case of an Allied attack on, say, the Kurile islands, in addition to one in the Solomons. This is an idiotic deployment for an offensive operation against Australia, but a logical one for a general defensive position. During the battle of Guadalcanal, Japanese naval units were deployed piecemeal into the area to stymie Allied attack, rather than en masse. Again, hardly something an attacking force would do.

In the end, neither the Kokoda trail nor Guadalcanal stopped the Japanese from attacking Australia: Japan simply had no way to do it, given their deployment and logistical situation, period. So, it logically follows that we should thank Japan for stopping Japan from invading Australia!

Sources:

Hara, Japanese Destroyer Captain

Sentai Sosho, Official Japanese History of the War

Grace, the Naval Battle of Guadalcanal

Collie, Marutani, The Path of Infinite Sorrow: The Japanese on the Kokoda Track