I got no idea on this, you will have to speak to your law firm. From my own experience, there are many instances whereby you need company stamps, eg: applying for company bank accounts, registering VAT, SSO and of course certifying documents for submission to authorities.
It's free of charge for cancellation at the immigration. Since you will be working here, I believe it's best just to follow through the correct procedures.
Why would you choose not to cancel it ? Anyway, it's a different Queue (N2) from your usual extension of stay (N1) at Chaeng Wattana. The process is pretty fast, I did mine within 1hour.
Yes. You are required to cancel your ed visa before you leave. Please speak to your school and get them to prepare the documents for you to visit the immigration.