Hi drewy,
A topper can be an effective way to soften a mattress that is too firm but is not an effective way to “fix” a mattress that is too soft, has soft spots, or is sagging.
There is more about this in post #4 here.
Coils and innersprings are the same thing (innersprings are made with a series of steel springs or coils). There is more about innersprings in this article and in post #10 here.
Just in case you haven’t read it yet … post #1 here has all the basic information, steps, and guidelines that can help you make the best possible mattress choice.
Phoenix