The Commission of Inquiry has no jurisdiction over non-Australian citizens and cannot summons religious leaders from other countries. For their part, even if they wished to appear, the GB could not - although theIr organization's official literature can be included.
I think citizenship is irrelevant.
The RC could invite or summons whomever it wishes to appear before it, although the compulsive nature of a summons can only be enforced where Australian police have jurisdiction making it pointless (rather than impossible) to summons someone outside Australia. Whatever their citizenship.
And it'd be shameful to refuse if invited.
If the witness does not travel to Australia, video conference was proposed for Applewhite to give more evidence from overseas, and the Act even allows the RC to take evidence overseas with permission of the other country.