So we need to add multitasking controls directly to the visible UI. That means they could be accidentally triggered much more easily, so we'd need to do that in a clear and obvious way that isn't confusing, doesn't lead to accidental triggering and is easy and obvious to get out of. Also they need to be unobtrusive and minimise the impact on the display of applications and content.

Clearly having obscure gestures trigger functionality isn't ideal, but are the alternatives actually better, or even viable?

I do agree with Gruber, the current gestures and behaviours are awkward. I can think of some alternatives that might help. One would be to run your primary app, then have a Control Center widget activate a multitasking layout, then it display the home screen as an overlay, so you can choose any app to go into the 'second slot' of the chosen layout. That may well come with negative tradeoffs, but at least the app selection/launch process would be familiar. There may be things that can be done.

However there are no magic wands here. Getting rid of gestures sound great, but then what do you replace them with?

A single gesture from one side of the screen, in the style of the Windows 8 charms? A set of buttons (side-by-side, floating, switch foreground/background floating etc).

Or put it on the dock. One gesture to learn - show the dock, multitasking actions.