You can claim that white nationalism has penetrated the White House all you want, but there is no evidence of this being the case.
That it has been more pronounced than the post-Southern Strategy GOP baseline in the Trump Administration is also quite evident to both opponents and overt supporters of White nationalism.
While I do think Andrew wouldn't approve of what Bannon has done recently (and many of his old colleagues do not), there is no evidence his views have ever gone into "extreme white nationalist" territory.
Also, if Steve Bannon is a white nationalist, it seems weird that Trump would support him, considering that presumably makes him want to get rid of parts of Trump's own family(Ivanka/Jared and their kids).