Can an employer rescind an employment offer due to not being able to start on agreed date due to injury sustained prior to starting date. All paperwork was signed and submitted to HR and I was offered a permanent part-time position..
This, yes.
Or, in the alternative, perhaps they are no longer certain that you will be fit for the work you were hired for, at the time they need you to do it.
That is what I believe that they think I will be unfit for the heavy aspect. Should I seek compensation from my first employer for losing future earnings due to the injury sustained