Hi Trevor,
Let me first mention that I've never shot a fashion show before, heck I don't even think I've been to a fashion show before, but I have seen them on TV.
I do shoot a lot sports and action though! I have a 7D, MKIV and 1DX and mostly shoot in AI Servo over 90% of the time.
Lets go over a few points first. AI Servo works well in low light but it suffers in very low light with low contrast targets, so hopefully Canon will make sure that the show will be lit up adequately, so as not to embarrass all of their Canon Shooters there. If AI Servo fails then I would switch to One Shot and just keep re-locking focus on your target prior to each shutter press, however you could still use high FPS though.
Ideally, Zone and Automatic focus should work, however Canon doesn't live in an ideal world. Therefore, I would recommend not letting the Camera select the AF points for you. I would recommend to manually select Single point focus with either 4 expansion points or 8 surrounding points. Also, do not use spot AF. You should aim your AF point array for a contrasting point on the model, like a hairline, neckline, or contrasting clothing. You should move your AF points around for composition which allows the model room to walk into the frame. So, if the model is walking from right to left than start your AF point at the 1/3 point from the right side of the frame and move your AF points around as you see fit. I would also register an AF point that corresponds to portrait (vertical) framing as you'll frequently be shooting in vertical, especially when the model poses at the end of the runway. I'm not a big fan of face detection either, because personally I wouldn't trust it. If you do experiment with zone AF, then make sure you select the smallest zone AF choice. I would never use 61 pt. detection in such a busy and cluttered setting.
I think case 3 will be too aggressive and twitchy for this fashion show. After-all, it is designed for a subject that quickly enters the frame, per Canon's example, " An Alpine Skier bursting over a hill and suddenly appearing in your viewfinder or at the end of a bike race when you want to quickly switch from one cyclist to the other. Hardly sounds like a model walking down the runway.
The tracking sensitivity and Accel/decel is also too high in case 3 and if one model crosses in front of the other than your focus point will jump to the new model that you're not tracking.
I would go with either case 1 or case 2. Case 2 will ignore obstructions slightly longer than case 1, since case 2 is less sensitive. Also note that tracking sensitivity doesn't mean that it will take any longer to acquire a new target. All you have to do is release your finger from the AF button and re-lock on your new subject and case 1 and case 2 will focus just as quickly.
The 24-70 and 70-200 should work well, unless you have faster primes. You may also want to bring a flash if you have one. I also agree with your default 1st and 2nd image priorities.
You should also use Manual WB. I would shoot in manual mode for this and set your SS and aperture and use Auto ISO, just check your histogram to see that your exposures are adequate, if not select a wider aperture or adjust accordingly. I would also try to achieve a SS of at least 1/500 if possible, or you'll start introducing motion blur.
Good Luck,
Rich
