Looking for help with this, the only thing I found was a two-year-old thread with information that is now outdated because of Update 18, so welcome to the new guide for the Lords of Dust chain epic crafting.
First of all, in update 18, the devs changed the drops like so:
Shards only drop in the epic end chest of Spinner of Shadows.
Seals only drop in the epic end chests of Lords of Dust, Servants of the Overlord, and Spinner of Shadows. They used to drop in optional chests but no longer do.
Scrolls can drop from killing any monster in the three quests.
A special note for Spinner of Shadows: In doing this quest, it was actually a rare occasion that at least one seal or shard didn't drop. On average, we had two either seals or shards drop. I've also heard of a super lucky person who pulled both a shard and a seal on the same run. This leads me to believe that the 15% drop rate for EH applies to shards and seals SEPARATELY.
Seal and shard drops used to be bugged so that they only dropped on EN difficulty. I can tell you from personal experience that this has been fixed, although I have questioned if the EE drop rate is really higher than EH. Anyway...
Here is where things get interesting, and here is where I hope others will contribute their experience. In farming for the ingredients to craft an Epic Envenomed blade, I ran Lords of Dust 20ish times and Spinner of Shadows probably 40 ish times each on either EH or EE (yay for fixed drop rates!), usually with 5-6 players in the party to increase odds for ingredient pulls. I noticed some patterns that made me wonder if certain seals only dropped from certain quests (like in the Carnival chain). These are the seals I noticed, and I'm hoping that others can weigh in, so we have a better idea of where to go for which seals. Remember this is the experience of only one person, and it's just a theory. If I'm wrong, I want to know! Please include what you have seen in Servants of the Overlord as well!
Three of envenomed blade in LoD, but not one in Spinner.
Everything "Templar" in Spinner (I saw five for Templar's Retribution!), but not one for the "Templar" items in LoD.
Cruel Nobility in LoD, but never Spinner.
Luminous Truth in Spinner, but never LoD.
Doublecross Bow in Spinner (only once), but never LoD.
I want to say that I saw a seal for the Rhakshasa Hide and the Darkstorm Helm in Spinner as well, but I'm not entirely sure. They may have been shards. I've never seen a seal for Bow of the Silver Flame, Silver Slinger, Fury of the Flame, Golden Guile, or Flameward.
PLEASE remember that to keep information accurate, you need to only use your experience since Update 18! (May 22, 2013) That's when they redid all the drops for this chain. Happy hunting all!