Where did I say examples are sufficient for understanding?
Yes that statement is too stong. In mathematics examples have value, but that value can be exagerated. There that is better. In addition care should be taken to chose diverse examples in order to prevent limiting ones imagination. In some areas it is nearly very difficult to grasp the many diverse elements a set can contain.
Xi have a common structure, which is embodied in X. This structure can be discussed without reference to a particular example. The idea of a function can be discussed without talking about a particular function like cos or sin, and I'm sure you know that this is a very useful thing to do.
A good point. Sometimes the trouble lies in thinking of x as potentially any element of X, sometimes the trouble lies in not thinking of some of the possible values.
If you can show (not assume) that Xi is a vector space, then various theorems and results from the abstract theory vector spaces become available for application when dealing with Xi.
That is true, but presumably the more general results must be proven before use. More general results are often more difficult and usually appear more difficult. For this to be worthwhile we would want to know that the more general result will be needed soon, or that it is available for low cost.
Is it easier to learn about the derivatives of each function seperately, or to introduce the notion of derivative in context of general functions and then apply the results (product rule etc.) in each case?
Then you better convince yourself that there are before you start studying X.
or never study X
I have no idea what that means.
One may consider the new examples to be 'pathalogical' and as such unworthy of consideration.