The Titans knew that the Shadowlands existed, but knew little to nothing of them - their concern was the physical universe, bringing order to chaotic worlds, and finding and tending to the maturation of their own kind (e.g. World-Souls like Azeroth herself). They were creatures of the universe and concerned themselves primarily with it. Insofar as to how the Titans compare to the Eternal Ones, the pantheon of Death, we don't really know. Some of have claimed the Eternal Ones are slightly more powerful, where others have said their powers are the same or at least closely matched. I'm inclined to believe the latter, but the real answer is we don't quite know yet. Insofar as individuals go, I would say the Jailer is to the Eternal Ones what Sargeras proved to be to the Titan Pantheon, the most powerful of their number and a creature of evil intent. In terms of importance I would say the Arbiter edges out Aman'thul as the Arbiter has a critical role in the functioning of the metacosm of Warcraft, whereas Aman'thul while imporant doesn't necessarily serve a critical role to the function of the universe.
The Void Lords, and the Void itself, stand apart from Death - and has directly attacked the Shadowlands in ages past. All of the primordial metacosmic realms sort of stand for themselves, and the physical universe as the "center" of the metacosm is connected to them all in a variety of ways. As for the connection of the Legion to the schemes of the Jailer and the Shadowlands, this is likely (though again not verified) connected to the Nathrezim and their likely connection to Death, the text called "
Enemy Infiltration - Preface" which seems to implicate the Dreadlords as creatures of Death involved in an elaborate plot against all the other realms of the metacosm. If this connection is true, then that would go a long way toward explaining how the Helm of Domination and Frostmourne wound up in the hands of the Legion Dreadlords, which was then passed on to Kil'jaeden and into the hands of the Lich King.