Is it possible Slash and Duff are simply "filling in" for BBF/DJ/Tommy?

[Sosso]

No. Mainly because most of the people who left, left because SLash and Duff came back.
At least that's how I see Tommy and DJ departures.

No, not really. DJ left because of SIXX::A.M. and Tommy left because of personal reasons - this has nothing to do with Slash and Duff.

[Wooody]

That's the official version. I think there is more behind the scenes.

[Thorned Rose]

As of now, it looks as if Slash and Axl got cool with eachother, and then by good measure Tommy and DJ kinda bailed.

I don't think they are filling in. I think it all just kinda worked out.

What was Axl going to do? get hired guns for the hired guns, that replaced hired guns?

At some point it's just silly.