if we define an error message in an asl code, like :

define game <...>
/.../
error <badcommand; txt etc.>
/.../
end define

then opening it with QDK and saving it again will remove this new error message definition in the code.

...and the same goes for comments (') in the game definition : if we enter the code manually, and reopen it with QDK, this comments is erased.

I will take a look into the first problem.

QDK ignores comments when loading a game, and so it will save your game without comments. QDK doesn't store the game in memory as ASL, which is why if you resave ASL in QDK you'll notice things may have moved around. This means there's no reliable way to keep track of where comments should be. If QDK read a comment that was about a particular line of code, it may end up being saved somewhere completely different. I can't think of a good way around this problem. If you need to combine QDK with manual ASL coding, I suggest you store any comments as QDK notes. Off the top of my head the syntax for these, if you're writing them manually, is

'!qdk-note <Note text>