That is a very sad story Katie.
852cM is substantial. If you have all six of those birth certificates, you have definitely been thorough.
The only thing I can think is that either his birth was not registered, or the name your Australian
contact gave you was incorrect (accidentally or intentionally) or he was registered under a different name. That doesn’t help you at all I’m sorry, and I’m sure you would already have considered those plus other possibilities.
It may be a case of holding out for some DNA matches.

Have you uploaded your raw data from Ancestry to other companies and Gedmatch? If not it might be worth doing so. I suggest all of you who match on that line also upload to all sites.
I’m not sure about other sites, but My Heritage has an Autocluster tool, which groups all related matches which may be of use. Unlocking the tools incurs a fee though but only one of you would need to pay the fee to see all others who matched with you.
It is already complicated enough, but might it be possible that you and your Australian match are related through a different line? Grandmother seems the obvious one from what you say, but is it possible you are looking for an illegitimate child that she didn’t have? Could Robert be related to another family member?
I’m sure you’ve seen this:
https://dnapainter.com/tools/sharedcmv4I’m confused by it, but I’ll leave it with you.
