You need to get it in writing from your employers as to exactly what these light duties are, so that you can hold them to it - verbal is not enough, don't return to work until you have received that in writing.
Employers should take notice of anyone returning to work, and ease them in gradually, or - IF they have light duties to offer - they can offer light duties. As I understand, there is no obligation on an employer to CREATE light duties for anyone if those tasks are not already there - the option then would be for the affected employee to take ill health retirement or similar, if they are not fit to do the full job as described.
I don't want to sound harsh but if you leave at the assigned time and that leaves staff members by themselves, that is not your problem, that is a problem for the company. You MUST stick to the agreed hours - your working all those extra hours isn't helping your case, and it could be argued that you don't need the light duties/minimum hours because you're still working the long hours. It's you that you have to be thinking about in this situation - what's best for you, not your colleagues - it's hard, but you have to consider your long-term health.