1. I am assuming you would have chosen the NOC code for your work ex based on your current job. Hence, you would clearly need to mention your roles and responsibilities for your current job as these would be matched against the roles and responsibilities for the chosen NOC code. Hence, providing a roles and responsibilities for all designations together under one head does not sound like the best option to me.
I myself have held 4 different designations in the same company and have provided details (dates, roles & responsibilities) for each of them separately. In my opinion this is the better option.
2. As long as you have held all these designations in one stint at the company (same employee number), a single entry would suffice. But please do mention all the designation with dates.
However, if multiple stints, i.e., working for some duration in this company and then resigning. Rejoining the company again at a later point. Then, each stint needs to be mentioned as a separate employment with appropriate NOC code for each one.
The above is based on my own opinion and discussions with other applicants. I have followed the above advice itself for my application.
Hope it helps