400 meters 19: I removed a block from the ceiling and made some small skill changes.

One minute 10: I congratulate you on your very ingenious solution! But that's not what was intended. So, I made the big tree bigger.

200 meters 5: I removed a few blocks from the ground, which makes your level solution obsolete.

400 meters 19: Besides an unimportant design element, I changed this: one glider and one slider are "in pick-up skill" and one shimmier has been replaced by a glider (so the two sliders are no longer assigned to the entrance).

One minute 7: I added two steels to the bridge.

I also made a correction on the next level, because the solution you found wasn't the intended solution. I wanted to fix the problem earlier, but I forgot :

One minute 10: I made the trunk of the little tree thicker.

100 metres:
9: Entirely valid alternative solution.
15: Huge backroute : I raised the water a little and added two blocks to prevent the 3 stairs from going directly to the other side.

200 metres:
5: Backroute: I removed the pick-up skill (which was a red herring !)
9: Slightly different solution than I had considered. Well done !
17: Solution very slightly different from the one I had intended (at the beginning).
18: Little backroute : I moved the stone block so that this solution no longer worked and the glider could no longer land on it.
19: Little backroute: I removed two shimmiers. More interesting solution than mine.

400 metres:
1: Well done! Very close to the intended solution. As Swerdis wrote, I don't like miners who stop miners, which explains the fact that I had two slider skills.
4: Backroute: One-way-wall added.
5: Well done. I put a lot of skills so that we can consider different possibilities of solving the level, but all of them will undoubtedly be close to the one I had imagined.
6: Little backroute: I added an unlock button and moved the pipe block by one pixel.
9: Huge backroute : A steel added, a one-way-wall added, enlargement of the hole and one less basher. Nevertheless, it is still possible that there are backroutes on this level...
11: Little backroute. Nice alternative solution. However, I lowered the "rocks" to prevent one from being able to pass underneath them (as well as in the 100 metres 13).
14: Little backroute: Close to the intended solution. I didn't think you could stop a basher with a blocker and release the blocker at the same time. I learned something. So, I removed a bomber and increased the number of lemmings to save ("29" instead of "28").
16: Backroute: I changed the number of lemmings in entrances #1 and #2 ("3 and 1" instead of "2 and 2") and raised the hole on the wall.
18: Well done! Nice alternative solution. But... more complicated than the intended solution.
19: Huge backroute : Steels added.

One minute:
4: Nice alternative solution, but more complicated than the intended solution. There is an easier way to solve the level.
6: Nice alternative solution.
7: Little backroute: I removed the blocker because it makes solving the level a little too simple.

For the other levels, your solutions are those that I intended, or very close.

Marathon 23: Well done. Indeed, the two pick-up skills are red herrings. But it's my fault, I shouldn't have added them. To fix the issue, I added two "steel" blocks.

Marathon 24: Good analysis. Solution intended.

Marathon 25: Once again, good analysis of the level. I like to imagine puzzles with the gilder and the floater (the level Tic et tac, for example). As these are permanent skills, you can't go back by assigning a parachute to a glider, and vice versa. This can become a real headache. I had a lot of fun designing it, this level.
Furthermore, I suggest you look at my solution, a little simpler than yours!

Marathon 26: Yes, well done!

For levels Marathon 17, Marathon 19 and Marathon 21, it's perfect, you have found the intended solution.

Regarding Marathon 18, I fixed the problem. In fact, this is a level that initially used the "freeze after 10 seconds", which made it much more difficult to execute, with very precise timing. Your comment (mentioning the locked door), made me smile.

Marathon 20: This apparently complicated (or convoluted ) level is not that difficult. In its initial form, you had to find the keys hidden behind the Egyptian numbers between 20 and 100, which was quite difficult for those who did not know them.
But in its current form, you just need to carefully observe the reliquary and the indications on either side of the exit: first, you have to bash the "waves" block and second, you have to bash the "beetle" block. In addition, you have to bring back the four keys and find the heart (it's easy, you have to bash the "heart" block), the only organ that was left in the body of the deceased before mummification.
Okay, it's not very obvious !

For Marathon 22, I chose to limit the number of builders to prevent the level from being able to be solved like this, but congratulations, I now challenge you to complete the level with only 16 builders. With good timing, it is doable !
You will find my solution in the attachment, because I don't think I have already shared it on the forum.