Does anyone have any experience using an Emobile SIM on a foreign iPhone?

I travel between Australia and Japan. I already have an Hwawei android phone on the Japanese Emobile network which is great for tethering my iPod, and for use in Australia (as the SIM is not unlocked - one of the only in Japan) but not much else. The Emobile service area is passably good, but the phone itself is a little low spec to be useful. I like the iPhone and hope to buy one as a replacement if I can do something similar, but don't want to jailbreak.

I can see a few potential issues:

a) SIM issues
Japanese sold iPhones are locked to the Softbank network, so I would not be able to use in Australia. By buying an Australian iPhone (not SIM-locked) would I be able to use the phone in Japan also, using my Emobile SIM?

b) Tethering
I understand that the Australian sold iPhones permit data tethering. Emobile also permits it on my Hwawei machine at a flat rate, and this is important to me. Would I be able to tether the (Australian) iPhone in Japan, if I used the Emobile SIM in an Australian iPhone? I don't want to jailbreak.

c) other issues?

Any thoughts would be greatly appreciated.