No risk to the mirror whatsoever. What you will need to do is either mount the adapter in the sideways orientation and live with some darkened sides that can be corrected in lightroom/aperture/photoshop or mount the adapter normal but bend the aperture lever towards lens centre until the second notch on the lower lever holder fully cradles it to ensure you don't score your plastic near the electrical contacts. Some people grind the aperture lever down but I do not endorse this approach, everything I promote is vintage gear safe and lever bending is fully reversible, grinding is not.
This darkening issue is only a problem with full frame bodies, the 1D 1.3 crop has no vignetting in the 90 degree adapter orientation and the 1.6 crop bodies have no interference issues.
Your lens will only stop down the aperture if it has the adapter mounted and the lever engaged. Simply flicking the lever without internally rotating the FD mount will not initiate the diaphragm. If you are in a hurry to see it you can use a small screwdriver to trigger the little unlock switches sticking out and rotate the mount. You are in Australia and the registered mail shipping there generally takes 2-3 weeks. I looked into quicker methods for international but the cost was above 70 bucks.
Edited to change New Zealand to Australia (same difference eh. Just kidding, its like saying Canada and the US are the same, would not go over well in either place)
Is there any risk of the 5Dmk2 mirror interacting with the adaptor for the 600 f4.5? my lens arrived yesterday and is in mint condition glass is all clear, no paint missing other than a little under the foot. the lens cap is badly perished and needs to be replaced and the case is in good condition, it came with a UV filter, I amssuming that the apperture needs to have the adapter engaged before it will actually operate? becasue without it connected to anything there is no change when turning the aperture ring. also when pointed down the focus moves probably due to the element weight. So overall pretty happy with the condition of it. Now just waiting for the adapter to arrive so i can play with it. I've put the EG-S precision matte focus screen in the 5d2 so will see how it goes