How is "reaching for the top surface" different from reaching to click and use the scroll wheel on just about every other mice? And since just about every other mice is thicker than the Magic Mouse, wouldn't this aggravate the problem you're describing since these are more elevated?
I'm not saying that most mice are good. Most mice are ergonomically poor.There are good ones out there, for example
http://www.aerobicmouse.com/, but only a few of them and with caveats. The issues with a touchpad are different from those of a mouse. The problem with Apple's mouse is that it's trying to be both. If it were just a mouse, meaning that the surface was not used for any purpose except perhaps a scroll ball, then it would be ok, though certainly not ideal. The problem is that they've given you a small area in which to move your fingers while maintaining the need to hold the mouse in place. That's different from a regular mouse where your whole hand hand rest on the mouse and you just use a finger to scroll with -- try it out on a small raised object to see how you'll naturally move your hand/fingers on a regular mouse vs. the magic mouse. The old mighty mouse allows you to place your hand any way you want around it and just lean a little more to click - not bad actually. I use it occasionally when travelling and I've found that it's good because I can constantly change the way I hold it, i.e. it's very flexible and doesn't force you into any position.
In any case, there are only a few good mice on the market today, so I'm not saying that Apple is the only bad guy. I'm just disappointed that they didn't do better.
As for that "tense gripping" you're talking about, technique #2 is achieved by NOT gripping the mouse firmly, it's quite the opposite as you have to loosely hold it so it can move between your fingers, instead of having to grip the whole mouse.
In order to keep the mouse in place while you scroll with two fingers you're going to have to grip in one way or another to keep it steady. Check out Cnet's preview for a good example of that.
How about trackpads on laptops? They have nothing for your palm to rest on and require a lot of finger movements, yet people with RSI will switch to trackpads or trackballs to alleviate the pain because they require less wrist movements. Gently moving the mouse using fingers require much less wrist movements and thick mice that fit your hand prevent you from doing that and force you to use your wrists all the time.
We're talking about both wrist and finger/hand movement, as well as shoulder and arm movement. All mice can cause RSI, but some are better for you than others, and even that will vary depending on the person and how they use their body while working at a computer. Laptop ergonomics are usually bad -- apple's latest multitouch button, for example, is far too stiff to be good for anybody's fingers/hands. If used in a relaxed manner, without hovering the hand and fingers above (thus tensing), the multitouch pad can be really good. But of course that requires the user to be aware and to learn a certain way of using it, and that's something most people won't do.
I suffered from debilitating RSI and spent almost two years researching and trying literally all input devices available at the time ( a few years ago). I found that there is no perfect device and that the best way to deal with RSI is to use multiple devices, take rests, have good posture and listen to your body. I primarily use a wacom tablet for input, as it allows my entire hand to be relaxed and my fingers to barely grip the pen. I also use the aerobic mouse mentioned above, but it's of terrible quality and keeps breaking down.
It'll be interesting to see what people say once they start using this thing. I'm going to go check it out this weekend...