1. If you are sending troops to China, but not giving them to China as expeditionary forces, then they won't be getting involved in combat as long as you are at peace. If you give them to China, you'll have to fight with Chiang to maintain effective control over them, and...well...you'll probably get slapped around by the Japanese. Japan starts with slightly superior tech than Australia, and their vastly superior industry will outproduce and outresearch yours easily, reducing you to at best an annoyance. At least they should, if they know what they're doing.
And since you aren't at war, I wouldn't imagine that the AI would "skirt around" your forces, since they don't pose any threat at all to them. Of course, knowing as little as I know about these things, the AI may be coded to watch for any number of units in a given province (whether they belong to a hostile or not) when deciding where to invade next. But if your troops are not putting themselves in any risk, yet you are scaring the Japanese away from certain key provinces, I don't see what could stop you from building a Great Wall of Aussies to block off all Japanese access to Southern China or anywhere you don't want them to go.
You don't want to be that gamey, do you? Do the right thing, send your soldiers to the Chinese as expeditionary forces (which is very, very gamey already), and at least let them
die for a reason.
2. There is no way for you to force Japan into war, at least not that I can think of. If I remember correctly, there is no event that would trigger a Japanese DoW on Britain, the Netherlands, the Philippines, or the USA in plain 1.06. Instead, a Japanese DoW would be hardcoded, and I seriously doubt you can trigger it. So...short of going into the savegame files, making yourself heavily Communist or Fascist, triggering a regime change, and declaring war on Japan yourself (dragging your allies in as well), you'll have to be content with building your military in preparation for that special day.
P.S. If you focus on Long-Ranged Submarines and Improved Naval Bombers, you just might defeat the entire Imperial Navy single-handedly, and then follow up with a conquest of the Home Islands. I've done it.