RE: Focus onto control that only appears on focus?

I can't think of any WCAG criteria that would disallow that. It is
sometimes used for hiding Skip links. Just remember that, even though the
thing is invisible most of the time, it is still taking up space on screen.
So if mouse users happen to pass the pointer over the control, they will
see the pointer change to show there is a link or button there. We see
pointers do that for no apparent reason on some websites, and it makes
users wonder what's going on! Poor UX but not a WCAG issue I think.

And if the user clicks on it it will do it's job, whatever that may be. A
UK university website I am working on at the moment had that problem. Their
skip link is positioned directly on top of the site logo, which is the
usual link to the home page. That's ok in itself (it has its own background
so is clear to read the link text), but you can see that they initially
tried to hide it, before focus lands on it, using opacity=0, because the
code is still there in the CSS. At some point they must have realised that
if a user clicked on the top half of the logo, they didn't go to the home
page as expected, but jumped down the current one! So they had to add some
extra markup to hide the skip link properly!
Regards,
Guy Hickling