There always are so many questions of the kind of "why not this instead of that" for these writing exercises. And many times, the answer is that there simply are too many ways to express one and the same meaning to account for them all. The list of "solutions" given is not to be seen as exhaustive. And if your answer differs from the given one, it doesn't necessarily mean that your answer is wrong. That's why these exercises are for self assessment. You're the judge.

Depuis la naissance de ma fille -- Since the birth of my daughter.
J'ai donné naissance à ma fille -- I gave birth to my daughter.

Here, the two sentences you quote mean different things (note: I haven't done the exercise, so I don't know the context).

Yes, you can use seulement instead of ne..que, but the latter is considered more elegant and "better" French.

Pour lesquels and pour qui are both correct, if they refer to a person.

Thanks for your reply Chris. I take note of the first two. 

For the 3rd question, it was in reference to the use of ce in place of cela (and not lesquels vs. qui).