As you are an owner you can just ask the Strata Manager for a copy of the exclusive use by-law.
Re this:
The previous lot owner, who had “exclusive use”, damaged the roof by hammering floorboards into the waterproof membrane, but every lot owner had to pay to get it fixed (and still do if there’s damage caused by us). Does this weaken our hold on the “exclusive use” area?
That is odd – that has nothing to do with exclusive use. If an owner does something that damages common property, or adds something to the common property without permission, then as a starting point they are responsible for that damage or addition. This is taking into account we don’t know what the by-law says, but I doubt that the by-law says that the previous owner could install something like floorboards on top of the roof. Similarly if you now caused damage you are responsible, not the other owners.
In terms of “fixing” the by-law, you could put a motion to a general meeting to revoke it, but I assume you want to continue to have exclusive use. I suggest that you first get a copy of the by-law and see what it says.