Okay so to try and answer some of this lets begin with the holds. From what I understand the holds are essentially raw magic. That magic is aspected to various forces but in a very general and unorganized fashion. The holds also appear to be able to pump out bigger bangs, so to speak, than most warren user can. That being said there does appear to be greater risk involved for users of the holds.
As for the warrens, and this whole paragraph is going to be spoilers all btw, krul seems to have understood that in order for magic to not be mostly destructive it needed to have an organized base system to really delineate the aspects. The newest forge book will likely give us a ton more info as to how the dragons helped krul make the warrens btw. To continue, it appears that the warrens allow for a much more directed form of magic imo. You can directly effect thoughts with certain warrens and heal grievous wounds with another but it does not appear that you can do both with only one. In order to form this organizational system krul offered up his blood. From that I actually take it to mean he sacrificed his circulatory system, in a sense, to create paths from Warren to Warren and world to world.
As for the dragons and the opposing forces thing I am hoping that the new forge book goes more in depth but I will give it a shot with some speculation. I am rereading the series now and am on dust of dreams. The otataral dragon is mentioned and talked about often and gives us some context clues about how things work. First dragons hate da other, for the most part, silchas ruin explains that the blood of tiam essentially drives elient and soletaken elient to fight if they stay to close for to long. They are opposing forces by their nature. When krul came to the dragons for help building the warrens he knew this and likely chained different dragons to opposing elemental forces as a balancing system once again trying to maintain order and help tone down the raw power.
I hope this helps somewhat, I apologize for spelling and formatting issues I am on mobile.