In dryos cameras, we essentially copy the canon firmware code that starts all the standard tasks, and replace those whose behavior we want to modify (task is like a thread or process). Using a720 as an example, this happens in boot.c task_Startup_my
movieRecord happens later (in the same branch as the SDHC/multiparition stuff, which you could also ignore for the moment), but the idea is the same.
To not run the modified stuff, all you have to do is use the original function instead of the _my.
spytask is the main CHDK task, which is required for the UI to work. You'll probably want to get this running as soon as possible, although you could just blink leds at various points without enabling that. the kbd task is our replacement for the canon physw task, and is require to interact with the buttons, and also overrides the write protect flag and deals with usb remote stuff.
You are correct that everything will need to be done eventually, but debugging will be much easier if you only have the minimum modifications needed to get to a given point. For example, a trivial error your modified capt_seq could cause the camera to crash on boot. If you have all the capt_seq, movie_req and SDHC stuff enabled at once, tracking down the location of the crash will be more difficult. That doesn't mean you should work on all the bits that you need, but you should take the time to figure out where non-critical parts can be disabled.
