If you write code simply using an interface, then it's nigh-impossible to infringe on someone else's code implementing that interface, regardless of how internal or technical or particular or unstable that interface is. The provider and consumer are simply doing two different things, and are expressing two different ideas, even if you look at it under the abstract lens of SSO.

Instead, they would have to argue that your code using the interface infringes on either (a) the interface itself, or (b) some of their other code using the interface. The former case (a) has been greatly weakened in light of Oracle v. Google (especially in the driver context where you can argue for interoperability), and my impression is that the latter case (b) can often be minimized to the point that it also falls under fair use. Alternatively, the 'GPL condom' separates the ultimate user even further from any copyrightable elements of the provider's code.

The provider of the interface could try to make it so fiddly that it can only be used in one specific copyrightable way, but if it's so bad that even using a 'GPL condom' would infringe, then you could plausibly spin something from Sega v. Accolade.

In other words, I think the "generally accepted view" you refer to is extremely questionable, even in the context of kernel modules. People just steer clear of these hairy questions out of an abundance of caution, especially when their actions wouldn't net them enough money to pay for litigation. (And as it happens, Nvidia is netting tons of money!) And with a well-designed 'GPL condom', it would be a very uphill battle for the provider to argue that the ultimate user has infringed on some copyrightable element of their code.

I disagree, but I think this is a question that could only be answered in court (and probably over several court cases).

The end-stage result of your argument would be that copyleft licenses are practically unenforceable because if you just make a separate textual file and then make reference to internal APIs in order to make extensions then you're simply using an interface regardless of the implementation and thus not bound by the license. So the strongest copyleft license you could possibly have is an even weaker form of the MPL ("function-based" rather than "file-based", and one that you could trivially bypass).

As for GPL condoms -- personally, I think a judge would not look favourably on GPL condoms precisely because they look like pedantic rules lawyering. If usage without a GPL condom would be a violation then I would be surprised if a GPL condom made a lick of difference in the eyes of a judge. But I've been surprised plenty of times, and as far as I know they've never been tested in court.