Is the contract legal only with the signature of the project leader and no official seal of the company?
It can be operated, but the technology is higher. Although there is no seal, there is a contract signed by the person in charge after all. If the company does not recognize it, it will hold the individual responsible. Moreover, if the company does not object to the previous work, but accepts and replies, it proves to be the default. You can entrust a lawyer to handle it.