Cursed Earth you get by doing the Ritual. There is no division Sigil In Extra Utilities 2 so you do not have to worry about that. To get wither skeletons you need to make a dark matter warper and when given a piece of dark matter it will turn skeletons into wither skeletons. Don't get to close though :) It is all in the guide under Dark Matter.
You don’t need a Redstone furnace. Once you get the crusher from IE to make the grout you can smelt it using the porcelain melter and tank. The book will tell you what to do. You only need a few grout to get started and then you can use cobble to get your seared stone bricks.
Has anyone else had issues with the new questing book in the latest 2.7.2 Release? There is a new quest titled "New Quest" and it has an icon of a nether star. This seems to have unlocked more of the chain for me and also shows that I have completed some quests in which I know I haven't.