The point is that we are supposed to relate to Hashem both as avadim and as banim (and the Torah describes us with both terms). From the perspective of avadim, it makes sense to ask why one should try to make one’s master happy if those efforts will not be acknowledged through a reward or a lack of punishment. But from the perspective of a parent-child relationship, this is not a question, because in a normal healthy situation, parents and children want to make each other happy out of love for one another, even without considering reward and punishment. Now, obviously, feeling love for Hashem the same way we do for our close family members is not always easy or simple, but that’s exactly why doing a mitzvah shelo al menas lekabel pras is considered a big madreiga.
You really haven’t answered the question. See above.