This thread has a detailed discussion on the lady. Ultimately it is what you want to believe (I think ICE does this on purpose)
http://forum.malazan...20286-the-lady/
I will throw some quotes,
Quote
‘Convert you? You do not worship the Lady?’
She nodded, so serious. ‘Oh no. We are the Sea-Folk. We follow the old ways. Oh, the last of the priests seemed harmless enough until he tried to use the boys to satisfy himself. So we bound him and threw him to the Sea-Father.’
‘The Sea-Father? Oh, yes. The old ways.’
‘Yes. The Sea-Father. The Sky-Father. The Dark-Taker. The fertile Mother. And the Enchantress. The priests spoke against her the most. But we do not listen. We know the Lady by her ancient name. Shrikasmil – the Destroyer.’
Quote
‘Enough foolishness, Ipshank. I renew my offer. Be my Destriant. The power you will wield will be unlimited. Join me! I have found my High Mage. And my Mortal Sword – or should I say Spear? He awaits my enemies on the Stormwall. Together we will sweep these invaders from our shores.’
Quote
‘What’s with the chest?’ Lazar asked.
‘A fragment of the entity charading as the Lady,’ said Shell.
‘A fragment?’ Blues repeated, alarmed. ‘As in the other name for the Crippled God … the Shattered God?’
Fingers sat heavily on a boulder.
‘Shit!’ Shell stared across the dark waters of the small crater lake surrounding this isle, to the near-black cloud cover obscuring the night sky, without seeing any of it. All that strength collected by the Crippled God. Added to him! What have they allowed here? What further catastrophes may very well be laid at their feet? She shook her head in mute denial.
Quote
SUTH LAY IN HIS HAMMOCK AND LUXURIATED IN THREE CONSECUTIVE days of relative inactivity – other than repairing his gear, and the usual make-work of cleaning the vessel. He was on board the Velenth, a Roolian merchantman commandeered for transport. The reassembled Malazan expeditionary force was returning to Quon Tali, and Command had yet to get round to formally reassigning him, Goss, Keri and Wess. He lay, an arm over his eyes, and tried to sleep while the great mass of vessels slowly made its way back through Black Water Strait.
This quote seems to indicate that the expeditionary force is going back but I am not sure if that also implies that nobody stayed back. Maybe a small portion was left behind. A small force would be enough potentially considering that most of the land was destroyed.
In general, after the area is conquered most of the attackers are expected to leave on to other venues while some would stay back to hold ground.
Regarding Mallick's strategy, we don't know his original goals but from what happened we can guess at them.
1. The Stormgaurd are gone (A big fighting force is no longer a threat to the Malazan empire)
2. The Lady is destroyed or at least minimized (A potentially powerful god/ascendant is out of the picture)
3. The rogue Malazan force is destroyed
4. The stormwall is destroyed (Mallick is a jistal priest of mael. He probably knows something about the stormriders and could have made a deal with them)
5. One less crippled gaurd fragment to worry about.
6. No more blood sacrifices on the wall (A lot of people became victims of the wall). This is the primary goal of Greymane.
7. Incidentally Greymane is gone (For Mallick, another powerful ascendant is out of the picture)
8. The Mare Naval force is destroyed. It is mentioned that the Mare fleet was a prominent threat to the Malazan navy