(a) Yes. (b) No.
(a) The answer is yes, BUT.
The answer is yes if your employer required you to work from home. In Canada, there is a form for that. Governments love forms. The form you need here is a T2200. If you read the form and the background interpretations you will notice that your employer must require you to work from home.
During the COVID-19 lockdown, it might be a matter of semantics where you were required to work from home or allowed to work from home. I suspect that most employers will be happy to sign the form because it will allow the employee some deduction as compensation for the inconvenience of working from home. I would not recommend making a deduction claim without the form.
Also, a benchmark is often useful. The government required many of its workers to work from home. If the are being required then it is a small step to say that many other situations where people worked at home were also required.