Greetings and Welcome to the forum. I see nothing out of the way with the questions you are asking. One of us may think of a better location to move your thread to so you get more responses tho.
The essay question is
"how does one religious worldview give meaning and purpose to the lives of people through its rituals and sacred stories and writings?"
I can only answer for myself. Knowing that our purpose is to worship Allah(swt) in all we do, say or think has given me direction and peace. It has given me the strength to help me control egotistical thoughts and desires. It has given me the insight to understand that it is only by serving Allah(swt) that I can serve myself.
Islam is a bit different than most religions. They actually are no rituals or rites of passage, instead all of life is a ritual in the sense all we do is for Allah(swt) We do have 5 established times of prayer each day, but in reality we have just one prayer time each day and it lasts 24 hours.
The wudu we perform before each prayer time, although seen as ritualistic, is more in terms of common sense. Is it not logical to wash your hands before you eat? Although you may do that 3 times a day, do you see it as a ritual or as a logical thing to do?
how does Islam give meaning and purpose to your life?
It does so by giving direction. It makes us fully aware that we as individuals and responsible for our own actions and words. We know the rewards and punishments that await us and we know the road that leads to rewards. It is our own responsiblility to search out and walk upon the road that will bring us to joy. The choice is ours.
Is there any particular ritual or story that makes one Muslim or show one's "muslim-ness"?
One can become a Muslim in a matter of seconds. But, it takes us a life time and a conscious effort to be a Muslim. The only thing that can distinguish us is how we choose to live our lives.
The Shahadah "I bear witness that there is none worthy of worship except Allah, the One, without any partner. And I bear witness that Muhammad is His servant and His Messenger." is this in the quran or in any of your holy text?
I am still very much a newbie in Islam and have much to learn. I honestly do not know when, where and by whom the shahadah was first saie in the form we see it as now. However the basic concepts do come from the Qur'am.
Allah is He, than Whom there is no other god;- Who knows (all things) both secret and open; He, Most Gracious, Most Merciful.
Allah is He, than Whom there is no other god;- the Sovereign, the Holy One, the Source of Peace (and Perfection), the Guardian of Faith, the Preserver of Safety, the Exalted in Might, the Irresistible, the Supreme: Glory to Allah! (High is He) above the partners they attribute to Him.
He is Allah, the Creator, the Evolver, the Bestower of Forms (or Colours). To Him belong the Most Beautiful Names: whatever is in the heavens and on earth, doth declare His Praises and Glory: and He is the Exalted in Might, the Wise.
(The Holy Quran, Al-Hashr, 59:22-24)
If thou ask them, who it is that created the heavens and the earth. They will certainly say, "Allah". Say: "Praise be to Allah!" But most of them understand not.
(The Holy Quran, Luqman, 31:25)
Give this warning to those in whose (hearts) is the fear that they will be brought (to judgment) before their Lord: except for Him they will have no protector nor intercessor: that they may guard (against evil).
(The Holy Quran, Al-Anaam, 6:51)
Revile not ye those whom they call upon besides Allah, lest they out of spite revile Allah in their ignorance. Thus have We made alluring to each people its own doings. In the end will they return to their Lord, and We shall then tell them the truth of all that they did."
(The Holy Quran, Al-Anaam, 6:108)
But those who reject Faith and belie our Signs (revelations),- they shall be companions of Hell-fire."
(The Holy Quran, Al-Maeda, 5:86)
Muhammad is not the father of any of your men, but he is the Messenger of Allah and the Last of the prophets; and Allah is cognizant of all things.
(The Holy Quran, Al-Ahzab, 33:40)
Verily in the messenger of Allah ye have a good example for him who looketh unto Allah and the Last Day, and remembereth Allah much.
(The Holy Quran, Al-Ahzab, 33:21)