Is it illegal for a lawyer to take the official seal of the firm, open a letter to appear in court and a lawyer's letter to accept a case?

As long as the evidence is conclusive, this kind of behavior is definitely illegal.

Stealing the official seal for profit belongs to theft and fraud. Once verified, it is a criminal offence.

As a lawyer, this is really stupid. Not only are you likely to go to jail, but your lawyer's license will definitely be revoked and you can't be a lawyer anymore.