What you want to do is just throw asteroids at the palace and keep going until there's just a big hole... a lá Heinlein's 'the moon is a harsh mistress'. On that note just hundreds of ork roks coming in and
landing falling on it - in scores of waves, should do the trick.
As to reinforcements, who cares? the objective is storming the palace, noone said anything about holding it.
However what we're dealing with here, if the BBB can be trusted, is a complex the size of Western Europe... Pretty tricky.
How about the surviving Blackstones, plus or minus the Planet killer?
Virus bombing?
Exterminatus? That seems to sort out pretty much everything else.
If what we've heard about some of the stuff Mars has is true, the Adeptus Mechanicus could rip hell out of it...
A few of those weapons from the dark age of technology...
Bring back nukes! They turned Baal into a radioactive wasteland and fused whole deserts to glass man.
Open up a c-vector field and throw it down there. (I know, I know, they don't exist in 40k... or do they? What was that field the Arkan Land threw up in the Third War for Armageddon?) Send the whole thing off at right-angles to the speed of light. Bonus effect, having a palace land on them should put a stop to those damned higher-dimensional beings and their damned Brockian Ultra-cricket
.
Apart from chaos0xomega's Tau force and possibly the previously proposed orks none of your estimates are anywhere near even one million men. I must say that sounds pretty silly to me.
Of course one would THINK that you could teleport a few marine first companies right into the middle of the place. Strange that the Heresy force, with supposed 'siege masters' the Iron Warriors and the 'tactical genius' of Horus and whatnot would go for a ground-based frontal assault. How dumb can you get? On the ground you're vulnerable. Stay in orbit! And at least if you do drop, don't podfall in front of the ultimate gate, for god's sakes. How hard is it to get inside walls when you're deploying from space? If the defenders don't have the capability to stop you landing outside the palace then they can't stop you landing on it. Bloody fools deserved to lose.
Even better idea! Drop
the moon on it