In writing fiction, how do you know how much detail and which details to include?
Answering this question requires you to first ask yourself - "What is the point of my story? What am I trying to do and what do I want the reader to experience?" Detail is not added for the sake of world building, and a story does not require a lot of detail to be engaging and immersive. The details you include should have purpose and meaning. Furthermore, they need to be significant and concrete. Significant details contribute to the plot of the story and either have meaning in themselves, or they give meaning to something that would otherwise be obscure. Details also need to be concrete, and appeal to one of the senses.
If a reaction is favorable, does that mean the product will form quickly?
No. The favorability, or the spontaneity, of a reaction depends on the stability of the reactants versus the stability of the products. The kinetics of a reaction depends on the mechanism that makes the products. A simple way to think of a reaction is to think of driving a car from point A to point B. Whether or not you can, and will, go from point A to point B depends on things like if point B has what you are looking for, and if you have enough gas to make the trip. It is a property of your starting point and end point. However, how quickly you get to point B depends on factors like the routes available and the traffic that you may run into. This is a property of the trip or drive itself, and how easy it is to get from point A to point B.
What are key features or characteristics of cells that make them the basic building block for life, and allow for multi-cellular life?
The cell is a self-replicating system that is sensitive to changes in its environment. The cell's ability to respond to change in particular is a large reason why single-cell as well as multi-cellular life is possible. Furthermore, multi-cellular life is possible because all cell types are comprised of the same chemical components. Cells can have many different functions, but they all do them using the same tools (i.e nucleic acids, proteins, lipids, amino acids) This shared toolbox can also be thought of as a shared language between cells. Cells can communicate with each other and work together to maintain and regulate life. Furthermore, by communicating with each other, cells create a system by which different subsets can focus on certain functions, and the collective group of cells is able to perform functions that none of the cells could do individually. The theme of emergent properties, properties that come about because a system is greater than the sum of its parts, is an important theme for life.