1) 19 January, the day you serve the notice
2) Yup if two months then it would be 19 March. Make sure you get return form signed off to make it black and white that you had indeed returned all company items.
Send emails to management&supervisor to notify the progression of the handover ( this is for your own record)

3)If they hold off salary then issue complaint to labor department.

4)already answered above.