I'm wondering the same as you. One day, I went to share the link with some friends so we could play together. And presto. We were greeted with the "Marked as hidden from view or Item has been deleted." Wish they didn't cave into the constant trend of bullying a content creator unprovoked.

Refer to my original answer above. When an item has been hidden from view or may have been deleted (which in this case, it's definitely hidden), Steam will not show the webpage.

As the original collection has been hidden but not entirely gone from the workshop, all you have to do is to click either "Subscribe to All" or click the green plus button to subscribe to the levels one by one. The files will populate in your left4dead2 < workshop directory.

No.

No. The most I can tell you is basic troubleshooting steps. They come in different methods, but I also recommend trying all of these before seeking out extra help from the community.

1. Unsubscribe from all mods to revert back to basic gameplay.
2. Subscribe to all of the levels.
3. After waiting for them to finish installing, open the game and see if you still have the bugs. If it works now, you're probably subscribed to another addon (manually or through workshop) that caused conflicts with your game.
4. Just unbloat your mods, it's not that difficult.

1. In your Steam library, right-click on your Left 4 Dead 2.
2. Find "Properties < Local Files < Verify integrity of files."
I recommend doing this on vanilla mode.
3. Steam will find the files you're missing from your game and replace them.

1. If you're subscribed to too many mods, or those that replaces the same kind of files - That will cause problems.
If your game doesn't work to the point it constantly crashes, do both "Verify Files" and revert back to Vanilla mode.

If neither of these solutions work or if you don't understand how to read this, I'm not helping you with that. That's as far as I know when it comes to troubleshooting in Left 4 Dead 2.

No, you don't have to subscribe to every single pack. Each pack is simply a variant of the final version.

1. Director's Cut edition - Original pack that was hidden on the workshop. The chronologically final release.
2. Uncut edition - Contains longer stages, Hank interactions, and other snippets that weren't included in the final release. This includes the mission "1 Million Visitors."

Long story short, the original version of this mission is Unskippable in the Director's Cut. If you fail to prevent the zombies from entering the mall, you fail it and have to restart the mission again. You don't want to do that as it'll make your game session longer. This is why it was cut from the final version.

3. Uncut edition - Contains outdated scripts and randomization. Finale map comes from the Left 4 Dead 1 version. Includes custom weapons. May be the least stable but I had no problems with this on vanilla mode.

Read the instructions above, and exhaust all steps.
I'm not experienced in bug territory, so unfortunately, there may be communities out there that will help you.

That's something we're not helping you with, you probably just suck at the game.