"No other firearms available" is not part of the exemption text.
The government made it clear " until a suitable replacement can be obtained" however this exception was made along with the promise of a buy back to enable you to purchase said 'suitable replacement" which has still not been implemented.
If the cfc/CFO can take an application for the sustenance hunter declaration and approve a fee waiver based on that declaration, they can also provide documentation showing the government entity responsible for granting this exemption has indeed approved you as a sustenance hunter
I agree with you about what should be perfectly fine according to the law, but I think you would have a hard time arguing your case to a judge regarding the "need" to use your recently prohib rifle.
As to a suitable replacement, there's lots of $350 savage axis rifles for sale in suitable hunting calibers... when you look at this from a financial standpoint I believe it would be pretty hard to argue. If you can afford the tank of gas to get to your hunting spot you can afford a "legal" rifle. Sure, the law has exceptions regarding the use of your current prohib rifle, but it also has a permit available to carry a handgun for protection purposes, but good luck getting that approved.
I'm not saying I agree with this, but that's just how I see it panning out in front of a judge. Then you will spend thousands on legal fees, which also could have purchased a "legal" rifle... See where this goes?