In what publications is he done well? I can't recall ever reading anything about him outside of internet forums.
The comics that he was originally featured in, and the reason he is in copywrite limbo. My understanding, though it is likely flawed, is that GeeDubs hired comic book writers on a freelance basis, but the contract didn't specify that any characters and entities that these writers created were to be owned by GW. Cue issues.
God it's been a while since I read those comics, so take that with a sliver of caution, my memory might be making him more impressive than he actually was.
I don't recall if Malal himself is deemed copyright limbo (or his name), but enough of the characters and concepts around him got the axe because GW would essentially have to restart everything. And they more or less threw everything out the pram in a tantrum. His rebranding as "Malice" is to my recollection a recent thing, and outside of the few fanworks that care to try and play him seriously, seems to be limited to 40k now. Be'lakor has basically become GW's replacement in the Fantasy and AoS side of things.
(Yes, I feature Malice in my own work, I actually prefer that name, but that's subjective)
Alas, if you are the type to read fanworks, my experience has been that Malal is written as comically inept most of the time. People have taken the whole "cursed to fail at the finish line" and ramped it up to Thanquol levels of failure. The only thing he's missing is Brian Blessed as an excuse for his plans going wrong.
So is he a god of division, mistrust and conflict? Does this apply only to the other Chaos gods or to mortals alike? What does he offer to his mortal followers?
That's an interesting way of looking at Malal. It applies to everyone. While Malal and his followers focus first and foremost on the other Chaos powers, it does NOT make them an ally of the southern realms, it just means that the men of the Empire aren't in their way at that moment.
As for what he offers... pass. Like I said, been a long while since I found and read the scans of those comics, and I'm not going to pretend to remember what canonical gifts he gives his champions.