wxWidgets is a decent choice. Not all of its controls are accessible though. Most infamously, the wxHTML control (wx's own HTML engine) isn't accessible, but the web view control (using the OS's web browsing engine) is accessible if you futz around at the native layer to force the keyboard focus into the right place. Also, on Windows, the list view control is accessible but the data view control is not. Frustratingly, the opposite is the case on Mac and (I think) GTK.

I suggest you test with Narrator rather than NVDA. Disclosure: I work on the Narrator team at Microsoft. But that's not why I say this. The reason is that NVDA and JAWS have some ugly hacks that they can use to make some GUI implementations (particularly using Win32 with GDI for graphics) accessible even if they're not accessible by design. For details, do some searching on the term "off-screen model". Narrator doesn't have this, so if your product works with Narrator, you know it's really accessible.