Dredge mechanics and Omnicience Matchup's Ruling?

[slave]

Hi all,

I have a few questions re: Manaless Dredge (dredge in general really) against the Omniscience Matchup.
I've heard a few different opinions on this....

Let's say my opponent plays Show and Tell.
They play Omniscience, I play a Griselbrand.

They then play Emrakul.

Now I'm curious as to when is the soonest point at which I could pay7 life and Grisel-dredge away? Can I Pay7 and put this on the stack immediately after Grisel comes out?
If not, when is the soonest I can?

Likewise, I'm also playing Phantasmagorian.
I play that at any time I could play an instant/activated ability on my opponents' turn, even in my own end step, right?

And last question - should my opponent attack with me with their Emrakul, and I have Bridge from Below in the GY;
I sac my creatures to **annihilator 6** and then when I have satisfied that, my Bridge(s) trigger, bringing a whole lot of Zombies into the field.
Is this right?

Thanks everyone!