One way I can see Rio buying the entire project is buying 50% JV...

One way I can see Rio buying the entire project is buying 50% JV partner now to keep the agreements in place with the DOE loan and permitting.
Once the loan has been drawn down and they are operational for a period of time, then they could buy the remaining 50% which would require them to do a loan deal with the DOE or payout the remaining balance on the loan.
If Rio could retain the DOE loan as part of the M&A deal I'd say for sure they would but this has been going for 8 years now so Ioneer aren't going to give it away for 500 million as you suggested.
Ioneer are in the drivers seat at the moment not potential JV partners, they can take their time to find the right deal, it's not Star Entertainment Group.