Marginal cost will be close to baseload/long term supply contract prices the majority of the time in any case - as you say, it's only during demand peaks that that price increases.
One problem today is that enough of the baseload capacity is coming from gas-fired plants that they determine marginal price even for period of lowish demand, and even then prices will be high because the gas supply-demand balance imposes it, not the electricity supply-demand balance. In the long run, we're all dead. John Maynard Keynes
So yes thye will be able to finance nuclear plants - but it will cost them more than if they borrowed at the same rate as the French State. In the long run, we're all dead. John Maynard Keynes
Absent raiding by the Assemble Nationale, EdF's profits are theirs, yes? So the only thing they're missing out on is the opportunity to lend their money to the private sector as they borrow from the public purse. I suppose from a financial perspective, it's all the same thing.
I take it there's no chance whatsoever of France breaking EU rules to give guarantees?
How will they be financed anyways? 20 years or 40? Any news on the transition would be appreciated.