Antarctica wasn't ice-covered for most of the last 250 million years. In fact, for much of that time it wasn't even in a polar region. Remember, the continents drift around on the surface of the earth like corks (albeit slowly).
Somewhere I have a link to a good web site that explains this nicely (probably on my work computer). If I can find it, I'll post it for you.
I'm agreeing with you.
That's what I'm asking actually: If the crater resulted in a massive rise in magma up from the mantle big enough to to be detected by its local gravitational influence, but the (very thin) crust is moving extensively, then why can you assume the magma bubble (located under the crust, right?) is still co-located with the crater?