Kindly approach a competent and experienced service lawyer and show all documents e.g. appointment letter, Rx of the doctor, legal notice, service rule book, standing orders etc and give inputs in person.
In consultation with your lawyer you may decide your strategy.
You may submit a fitting reply to the legal notice.
You may collect all incidences and evidences to establish what you have quoted.
Designation alone does not decide employee is a workman or not.
Apparently you have touched the ego or raw nerve and now company has become adamant and recalciterant and announced that we will see you.The notice seems to be to step upon the toes of employee.
Charges are easy to level but may be difficult to prove.Companies feel that employee shall not be able to pull the employers/directors/MD/Chairman out of cozyness of their offices,in person to courts and their legal cell, lawyer shall represent.
Your lawyer may suggest that you are being defamed.
Defend yourself.