I need some HR advice

I can't speak to Canada regs.
And there are too many "it depends" to give much general advice.

I wish I knew something about Canadian HR, but I don't. I could tell you about US Employment laws and protections. In the US it can depend on how large the employer is and how long you have been working there.

Another pea in the U.S. Here.

I don't think it's likely they are doing anything considered wrong or illegal but don't know your areas specific laws.

From their standpoint:

I wonder if they can go the route that you were hired for the job/business, not location? (In the reassignment of the location)

Would you stating you cannot commute be you "refusing" the hours of "your job" from a HR standpoint to them? (Not giving you back the same amount of hours or shifts)

Would you not working at the assigned (other) location be reason for them to let you go legally?

I would be curious what the proper means would have been from their side. They have a job that needs doing, the employee hired for it has stated he or she cannot complete said duties of the job, and other employees cannot cover the responsibilities. Therefore they hired someone to do the job that needed doing.

Is it common there to hire temporary help then let them go after the employee returns?

In the U.S. There are so many variances. We have federal laws, some states have laws, I believe counties and cities in some cases may have laws; some sectors of business have laws ...

In the U.S. Employers that are large have employee protection in these situations in some cases (smaller do not legally but many still follow them).

For example if it were a large enough employer here they would have to offer you a "like" position upon your return. Here that could mean many different things. Definitely no change in wage or benefits but it could mean working different shifts on a different team, or even a different (reasonable) location that doesn't require relocation.

If an employee is critical, they may go from a manager to an employee after a leave, as an employer could state they are too critical to the operation of the business to go without (for something longer term like weeks or months) if others cannot cover for them. The same could apply to any member of a small in size team of a large employer.

The employer could hire the position but wouldn't be required to do it temporarily or to let the replacement go so the original employee can have the original job back. The employee would get a job back but the employer wouldn't be required to create an additional management position (or shift) for them to return to.

In my experience generally if the employer had multiple staff they would ask others to split shifts or job responsibilities while someone were out for medical leave. I have seen where people return back quickly on a lesser time basis to ensure no change in job responsibilities while on leave - but that's more a team situation not where one person is left alone to do a job as it sounds you are.

The above is general and slightly rusty as I'm not currently working HR in regards to this type of thing.

It depends on employment laws in your province and on your contract eg is there a clause saying they can change your location or restructure other aspects of your role? Is there some general duty of 'God death' dealings in employment negotiations?

I would find an employment lawyer.