Wow, what a month. Sorry to hear of your setbacks. It ought to be good to get home.

I'm a bit confused -- you say you are "entitled" to business class transport home as it's the end of your work tour, but the company is showing "no interest in providing a refundable or at least changeable fare". Do they want to fly you home right now, or not wait until you get a permission slip from the medicos? You say there is "no cost to company" but of course they are paying to send you home, correct?

Once you are ruled fit to fly you can have the company buy any ticket they want ex-CDG (presumably), but you don't know when that is. So... what is the question? I would send your ex-employer medical data indicating that you're grounded for the foreseeable, but you will have them book a J class ticket back to Oz when you're able to travel, etc. and at that point it can be a discount / non-refundable deal if they like -- which can be had on any number of carriers ex-CDG.