Of course they have to prove that they can payback the loan,...

I believe the DOE GRANT must be used for the purpose detailed in the submission. The negotiation is how it will be paid out (ie. What value and at which milestones).

I was under the impression that they needed to wait for the engineering works to conclude at Riverside as it informs the design at the new site which will underpin the capex and opex and thereby the sustainability of operations.

Of course they have to prove that they can payback the loan, hence the extremely arduous application process.
They will not have that information until the engineering works are done. Chris has consistently stated that they are looking at a range of additional financing options. Sure, a CR is possible, but they have a range of options they want to explore first. I am certainly confident that they won't be doing one until they are in production, which I expect will coincide with significant increase in the SP and would therefore minimise dilution if that is what they need to do.