To answer your questions...
1. In the level editor, go to the addin tab, and go to the levels element. There will be one ID that starts with a lower-case o, enter the ocd you want in there by entering the type of ocd first, then a comma and the number. ex: balls,29 | time,11 | moves,45
2. Be more specific. Are you talking about an island map? or how the levels are organized in the level pack?

Both. An actual working chapter map (Going Up, Cog in the Machine etc.) Like where in the actual goomod a completed level unlocks a new level, Ocd flags, Names, Level progressing graphics change (like the eye in your "Lost in Paradise: Feed Us")

Easy

First, you need a buttongroup. Id recommend copying one from another chapter from the original game. This is where you put all of your levelbuttons.

For each level button, enter the level name for the button ID (If the level name in the editor includes underscores and capitals, those have to be included as well) And below that are onclick, omouseenter and omouseexit. For onclick, enter pl_(Level name). Omouseenter, enter ss_(Level name). Omouseexit, enter hs_(level name). You need to do this for each levelbutton (Make sure they are in order in the buttongroup).

As for the pipes, they are scenelayers with a special ID format. Enter in the ID this: levelpipe_(level name) NOTE: enter the level name for the level AFTER that levelpipe.

OCD flags are ALSO a scenelayer, enter the specific ID format in ID: ocd_(level name) but now you need to add an anim to the OCD flag. The anim you should use is: ocdFlagWave. you also need to enter a animspeed, the animspeed could be any number, but I recommend it being in between 0.5 and 2. I also recommend giving your OCD flags animspeed variety. ex: OCD flag 1 with animspeed 1, OCD flag 2 with animspeed 0.5.

For the "level progressing graphics change" its actually just a levelpipe with different textures.

Hopefully this helps

Just follow this. http://goofans.com/developers/resources/chapter-addin-development

Thanks, but I have one more questions
Is there a way to add a button that counteracts gravity (via reverse linear force field)?

Not sure. I’ve never done it myself, I just know that it exists.

Actually, I think that's possible. The only thing is that it affects all non-static geometry.

All you have to do is add a linearforce field, set the force to double the opposite of the gravity, and enable geomonly. Make sure the gravity force has geomonly disabled.

For example, if the gravity is -10, the geomonly force should be 20.

does setting the button depth for 9001 make it stationary? I accidentally did that one time