Islam

The adoption of the crescent-and-star symbol for Islam occurred quite late in Islamic history, probably in the nineteenth century AD.

Most likely, this was due to European influence; because the cross symbolized Christianity, Europeans felt there should be a symbol for Islam. So, this symbol was adopted and became popularized.

So there is no inherent connection between the crescent moon-and-star symbol and Islam. However, crescent moons and stars have been used historically in Islamic art, on coins, and on banners as a cultural symbol. (They also appear worldwide on art in general.) 

Of course, the crescent moon symbolises the lunar calendar, especially marking the beginning of the month of Ramadan and Eid al-Fitr, so possibly this is one reason it was accepted. 

I would aver that the fundamentals of Islamic theology are actually quite simple. For instance: God exists and is omnipotent/omniscient, is just and good, and responds to prayer; there is life after death and we are judged in accordance with our actions and intentions; God sends messengers to guide people; the Qur'an is divine revelation; the Prophet Muhammad was the final prophet; there are other beings such as angels. 

It is true there are some arguments about the details of this (for instance, details of exactly what happens during the Resurrection), but I think the fundamentals are simple and this is one reason why Islam has spread so much around the world. Most Muslims will agree on these major points and disagree on smaller points, such as how exactly to do wudu. 

However, I also don't want to shoot down your question; clearly you are considering there to be many different opinions.

In this case, I would say that it is human nature to differ over time, especially when a community becomes large, and, over time, Muslims developed various worldviews and understandings, especially when faced with new questions. 

As for why God made it complicated, the Qur'an often emphasises the importance of using the intellect, pondering, taking lessons from the natural world, and so forth. So, perhaps we are meant to use our intellect and experience to find solid answers to the questions that are before us - that is, we are meant to put effort into it, rather than just receiving instructions like a computer. 

This effort is what distinguishes us from the angels: they see the divine realities directly and receive their instructions and do them, but they do not have to go through the struggle to find out what is true or not. Therefore, this ultimately gives us a greater potential even if the process is difficult.

Conversely, we value the things we have to work for (including theological and spiritual understandings) and do not usually value the things we are given freely.

Just a few thoughts - I am sure there will be others!