Which dual-platform clipless pedals allow for the least amount of movement between the pedal and sneaker
I have a Set of PD-M324's in my box of bits. Plenty grippy, to the point unless you are extreme down-hilling or free riding I doubt you need better. I have double sided caged SPD's with a cage similar to the look of the A530, also in my box of bits - no where near grippy enough, especially when wet. They are good for MTBing when you are unclipped alot - when learning SPDs or technical ground and lack of confidence, but now I am riding less technical XC I do not need them.      

Its not possible to make a grippy pedal that also has an SPD - how would you rotate to  release the clip?  Single sided pedals are a great idea that does not work very well most of the time for most people. Inevitably the side you want is facing down when you put your foot on the pedal. You cannot be certain, without looking, which way it is, so you cannot rely on the pedal when you need it quickly. They are OK for casual riders and might work acceptably for roadies who are not stopping a lot for traffic, but for me, and others I have spoken to who have tried them, they are just frustrating.  

I now always ride with my SPD's. If I was to start riding sneakers/street shoes, I would go to a caged SPD, and wear the slippages, but those rides would be down to the local store with the family. If you are not prepared to wear the slippages, go to proper platforms and discard the SPD's altogether, or swap pedals when you want to change styles. 

How badly do you want SPD's? Have you considered getting some SPD compatable sneakers/street shoes?  

If you do go ahead with the single sided pedals, I recommend the PD-M324, but consider something like the M647 (Clips both sides with cages).