Is there a way to access the code and element from a deployable package or DLL of a model?
You have just binary data created by compilation, not source code. You should be able to get any version of code from your version control system. And if you have a sane process to create deployable packages (using build pipelines), you can be sure that all code must have come from source control.
If you don't use a version control system, stop all other work and resolve this critical process bug first.
There is no decompiler from CIL to X++. There is a way to decompile it to some other languages, such as C#, but what you'd get would be very different from your original code, because of all the things that X++ compiler does under the hood.