Cryptanalys of "bad" RC4 implementation
This post answers my first questions: RC4 Keystream Reconstruction

Because I integrate others informations to this problem.

The RC4 algorithm has some well-known vulnerabilities but it is not completely moldy. So, if I understand correctly, finding the initial encryption key even with the clear text is difficult (Principaly in this case, no reset rc4, key lenght, ...).

On the other hand I understand that I work on a similar challenge.

I have to decrypt a file of the same type: INTEL Hex Format

So, Drop-N routine or not, if I know the plaintext I know the associated XOR key.