Because of the number of technical terms and also because I need to practice my French, I will answer in English. I hope that's okay with you.
The definition of “private interests” is the huge loophole in subsections 3(2) and 3(3) of the code. Subsection 3(2) says that it's only financial interests that are covered. Subsection 3(3) says you cannot be in a conflict of interest if you're dealing with a matter “of general application” or dealing with something that applies to you as an MP in the same way it applies to other members of the public.
This allows MPs to participate in almost all decisions and actions, even when you have a secret financial conflict of interest and will profit secretly from the decision or action, or your family members or friends will profit from it. Friends should be covered in the code in terms of their private interests and you having to avoid conflicts of interest. The reason for this is that 95% of the decisions and actions that MPs are involved in are matters of general application. It's a huge loophole.
Secondly, the code doesn't even apply to MPs' staff, to answer your second question, so they can do the very few things on behalf of their MP that the code actually prohibits MPs from doing—and it's only very few. It's only 5% of your decisions and actions that are even covered by the code. Staff can then do those things for you that you're prohibited from doing, because they're not covered by the code. Staff should be covered by most of the key rules of the code.
There are many other loopholes in the code that I wish I had time to talk about that make it the “almost impossible to be in a conflict of interest code”.