The employer cannot force you to attend a meeting with them now that your employment has terminated. They also cannot take any action, such as disciplining you. Whether you agree to meet with them is entirely up to you You may decide to see what the issues they have raised are but whether you do anything about them, like defend yourself or help with their investigation is for you to decide and if at any point you feel unhappy with proceeding you can simply walk away
yes, but depending on what you actually did it would amount to negligence and as a professional you could carry personal liability so whilst this is far from obviius as being the case here, you may wish to see what these errors are and provide a reasonable explanation rather then leave the employer to continue based on their own perception and you then having to explain yourself to others, such as your professional body
Please let me know if this has answered your query or if you need me to clarify anything else for you in relation to this?