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For other question, some quest items get stuck in your inventory. Find a console command on internet to delete it (if you completed the quest already).

You need to do the main quest until you get the Blackreach scroll. There is no other way (maybe console commands if you're on the PC, but you're on your own. I don't know if that'd confuse the game in this case or not). Yes, this is exactly why Dawnguard sucks so much and is indicative of many other reasons the base game was poorly made.

Also, that scroll says it weighs 20, but it doesn't. Quest items have weight values but, counter-intuitively, weigh nothing as long at they're stuck on you.

You don't actually need to do the Main Quest. The side-quest that rewards the Scroll can be done anytime after you're level 15. Simply go here: http://www.uesp.net/maps/srmap/srmap.shtml?centeron=Septimus+Signus%27s+Outpost - and do the quest. At the end of it you'll acquire the Scroll.

http://www.uesp.net/wiki/Skyrim:Discerning_the_Transmundane - this is the quest that actually rewards the Scroll. While during the MQ you will be required to complete it, it can be done way before that :)

The quest you need is ELder Knowledge an is recieved from Paarthunax.

If you just want to use console commands then open up the console and type player.additem 0002D513 (though as noted above that could cause probllems)

The librarian does give you information, he fetches two books for you. If you caught him in his bunk, he will walk to the library, fetch the books, and either wait there, or go back to his bunk. You may need to speak to him in the library, after you read the crazy book. Just open both to the first page and the dialog will be possible.

Ultimately he sends you to a crazy guy in a cave in the northeast. This guy sends you through a Dwemer ruin to get a scroll. Never mind the main quest tie-in and just do it anyway. Whenever you do the MQ part, you'll just tell the quest giver you already picked up the Elder Scroll, you both LOL and the quest continues on normally.

You can buy the Elder Scroll from him, I think, I'm not too sure on this. I know you can sell it to him when you're done. So maybe they did that and bought it back.

Are you on PC or console? If you're on PC, just do player.modav carryweight 99999 and never worry about weight again. Stupid metagame I don't care about and you shouldn't, either. JMO.

There aren't that many storyline quests.

If you don't know which are storyline, as i recall they're usually at the top of the list.

You can always use the wiki.
http://elderscrolls.wikia.com/wiki/Skyrim

I couldn't get in the castle, I was told to take a boat there which did, but the NPC following me won't do anything, nor will the catle gates open.

Doesn't quest items just pretend they have weight, but actually weight nothing as long as they are marked as undroppable quest item?

You dont need to complete the main storyline. That guy in winterhold tells you to find someone in a cave in the far north-eastern side of the map. And you then have to do his quest line to continue.