Is it permissible to write "unmarried" instead of "widow" in a CV, and is that considered a form of dissimulation (tawriyah) or an infringement upon the employer's rights?

There is no harm in using dissimulation (tawriya), unless it leads to taking something unjustly. For example, if a woman writes that she has not married in order to take money she does not deserve, then it becomes forbidden. However, if there is no such prohibition, then there is no objection to the aforementioned writing.