Level 13 is the one level I have regrets making, it is, in fact, impossible to beat in the current version sadly. I made it at the ~40 hour mark on around 5 hours of sleep and actually forgot to play test it. My friend actually caught the fact it was impossible but I forgot to go back and fix it sadly.
The one way blocks actually had their sprites changed multiple times and I agree with you that they are a bit too hard to see. I still wish I had changed them.
As for the puzzles being to complicated and difficult, I fully agree with you. One of my play testers called my game "The new worlds hardest game". If I had more time I would have spent more time on creating them and play testing them. Check points would have been a good idea for levels, or a some sort of ghost replay of your last run through so you can see what you did wrong.
I haven't made a full walk through yet but its been suggested multiple times so I should probably get on that. But for you, you're in luck! If you go to the games actual itch.io page there's a gif of me completing level 9. Thanks for the detailed feedback and I'm glad you enjoyed the game!