I alternate between single shots and focus bracketed series. I shoot tiny things near the round in macro. I try to set my focus bracket parameters in C1 Custom status, so I don't have to go into the menu on my hands and knees with bugs in my face and sweat in my eyes, while my target scampers under a log. Here's the problem: the Custom function has no stay power. If I switch from C1 back to Av for a single shot and then back to C!, focus bracket has reverted to DISABLE. The same happens, if I turn the camera off and then on again. This is not how custom functioning is supposed to work. Am I missing something?
Yes, I think you are missing something. Custom shooting modes are a preset, not a memory slot. You use one of the Creative shooting modes as template, which is REGISTERED as a custom shooting mode. Not only are exposure settings registered, but custom functions, AF Points and AF mode, Metering mode, custom function button programming, etc.
If you want to be able to retain your current setting when you turn the camera off, then DO NOT use a custom shooting mode. If you do, be prepared to tweak the settings in the custom mode to what you need for your current scenario.
That is how it works.
When the camera is powered off Focus Bracketing is supposed to revert to Disable. There isn't a way to change that behavior on our cameras.
For it reverting when you leave C1 mode and go back into it did you register the focus bracketing setting to the C1 mode? If settings haven't been registered to the custom mode it will revert to the default mode settings when you leave the custom mode. To do that put the camera into either P, Tv, Av, or M mode, dial in the settings the way that you want, and then press the Menu button. Go to the Custom Shooting Mode setting in the Menu and you will have the option to register the settings. When you select that you can choose the custom mode you want to register the settings to and that will make your current settings the default for that mode. That menu option is on different pages from camera model to camera model. Typically it is in the yellow wrench section.
Some settings like HDR and Auto Exposure Bracketing cannot be registered to a custom mode. For those settings you would need to go into the manual and set it up each time you want to use those shooting methods. The manual doesn't specify that would happen with Focus Bracketing, but it is likely it isn't a setting that can be registered since it is supposed to revert to Disable when the camera is powered off.