Here are some more hints without giving it away in full:

4 is part of the debug world "Chowdren" which can be entered from a secret room in the main menu

5, 8, and 9 are reachable from the main menu

6 is the save station you encounter when walking the path below the infinite darkness to the hidden fairy city

7, but without the rightmost screen of your labeling, is part of The Interlude. The map tells you how to reach it

10 is reachable through a hidden path from (23,20) in chapter 3. The entry spot might be obvious, but getting there requires advanced bouncing skills

11 is reachable from one of the adjacent screens and is actually part of the world surrounding it

12, including the screen above which you labled as part of 7, is an Interlude secret

13 is an Interlude secret as well, reachable from one of the adjacent map screens

14 is reachable from the screen above through a secret passage in the right wall

15 is the only real secret of chapter 1. Getting there requires quick reaction after you've left the temple where you stole the 2 candlesticks

2. In the script file "Secrets.ini", there's an entry titled "[Dead End]" that I haven't been able to get yet. Anyone know what triggers it?

Ah, I didn't think about that. It's not hard to fix though.

In case there still is a chance for an update to this game, it really would be a good idea to add those markers. The challenges are hard enough, so at least tell where to go