Hi awoods,
Yes … you can see the specs of the iChoice here.
No … it would be similar to any other topper and your mattress protector and sheets would keep it in place.
There are some good online sources for polyfoam in the component post here. Some of the big box stores or any supplier that has a good return policy would also be worth considering. Even something like this that can provide some additional softness under your pressure points and will provide a more “relaxed” and less resilient sleeping surface and a more “pillowtop” type of feel may be worth considering to see how it affect your sleeping experience.
Density is the most important factor for durability but with a topper that can be easily replaced durability may be less important than it would be if it was inside a mattress and couldn’t be replaced but I still wouldn’t use anything less than about 1.5 lbs because you could be looking at a useful lifespan of months in some cases.
How a topper feels and performs and the softness/firmness of a topper will depend on a combination of many specs including IFD, thickness, and compression modulus (see post #4 here) and this can become very complex so I would keep things simple because specs won’t tell you whether it will work or not so I would focus on the type of material in the topper (which will determine how it “feels”) and on the thickness (which will affect the alignment risk), and on the general firmness rating (firm, medium, or soft).
When you have purchased a mattress and more simple and basic “fine tuning efforts” (such as re-arranging layers, exchanging a layer, or some of the other previous suggestions) aren’t working as well as you hoped for then that means that you are “outside the averages” that would work for the large majority of people and trial and error and your own personal experience will be the only way to know whether a particular topper will work well for you in combination with your mattress and in these types of situations the return policy of any topper you purchase may be the most important consideration of all.
Phoenix