You really have to spend some time formalizing in your mind a philosophy of solo diving, before you actually go out and do it. The first step is to ask the questions that you are asking, to figure out how you want to approach soloing.
Some people approach soloing from a minimalist perspective, and take almost nothing with them. Others utilize a full redundant philosphy, and take two of everything. Still others take a graduated approach, something in-between. A lot depends upon the conditions under which you dive, temperature, visibility, weather, currents, depth, entanglement risks, etc.
YOU are the one who is taking risk. What level of risk are YOU willing to accept? Are you willing to depend on a CESA from 15 feet? 30 feet? 60 feet?
I'm not willing to depend upon a CESA whatsoever. That decision drives what gear I take with me when I dive solo.
Under what conditions are you willing to dive solo? Have you set parameters for yourself?
Some of my solo diving scares the piss out of me, and I have thumbed my own dives countless numbers of times. So I do emergency drills (such as my valve drills, mask removal & replacement, stage switching, etc.) at the beginning of each and every solo dive, which gives me confidence to venture forth by myself, especially when it's at night, the weather is bad, and the visibility is very poor.
YOU have to decide what YOU are going to do, before YOU actually go into the water ALONE.