No, they do not disappear. The collectables are not missable, you can get them all after completing the game.
There are 15 quests in the stormcloak storyline, two of them give you an achievement, four are optional, one is missable and the rest are basic quests.
"If a guy isn't calling you, it's because he doesn't want to call you." "Don't be flattered that he misses you. He should miss you. You are deeply missable." "You deserve the best and the best is out there looking for you right now."
No, all quests can be completed after the main plot is finished. There is never a case where a certain quest becomes impossible to start (or finish) at this point, all of them are available to you, once their prerequisites are met.
There is a missable mini-boss on Void Quest Chapter 7. If you miss it, you will not be able to open the door to the Mitsuo boss battle. You need to find and defeat the Killing Hand mini-boss on chapter 7 and get the Orb of Darkness from a chest in its room.
You need the ability soul, Rahab, gained by defeating the boss of that name. His location is the Subterranean Hell, and is actually quite missable since there's only one access point at a time when much of the castle is already open to you. But if you look at the map in the related link, you can guide yourself to his general area and boss chamber.
Machoke's Missable MovesSubmission (Lv. 32)Cross Chop (Lv. 40)DynamicPunch (Lv. 51)TM06 ToxicTM35 FlamethrowerTM38 Fire BlastTM39 Rock TombTM52 Focus BlastTM80 Rock SlideTM87 SwaggerHM08 Rock ClimbRolling Kick and Close Combat (Egg Moves)Mega Punch & Mega Kick (Move Tutors in FireRed and LeafGreen)
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 1 words with the pattern M--SAB--. That is, eight letter words with 1st letter M and 4th letter S and 5th letter A and 6th letter B. In alphabetical order, they are: missable
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 3 words with the pattern -I-SA--E. That is, eight letter words with 2nd letter I and 4th letter S and 5th letter A and 8th letter E. In alphabetical order, they are: kissable missable rinsable
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 1 words with the pattern MI-S-B--. That is, eight letter words with 1st letter M and 2nd letter I and 4th letter S and 6th letter B. In alphabetical order, they are: missable
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 7 words with the pattern M-S--B--. That is, eight letter words with 1st letter M and 3rd letter S and 6th letter B. In alphabetical order, they are: maskable mastabah mastabas miscible mislabel mislabor missable
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 6 words with the pattern M-S-AB--. That is, eight letter words with 1st letter M and 3rd letter S and 5th letter A and 6th letter B. In alphabetical order, they are: maskable mastabah mastabas mislabel mislabor missable