I know the basic difference between the two on their own is that Kevlar is more heat resistant + denser & heavier, plus stands up better against contact/point blank shots, while Dyneema isn't as heat resistant or stands up to contact shots as well, but is lighter weight and not as dense.

On their own, obviously you trade some protection for mobility with Dyneema. But between IIIA Kevlar & IIIA Dyneema, is there any difference when using either for a backer to a Level III/IV ICW plate, or will they do the job just as well?

Dyneema will be fine

Dyneema works great. However, when speaking of strength with Kevlar vs Dyneema, it's important to note which type of "strength" you are referring to. Kevlar generally has a higher tensile strength but as you go up the scale in terms of caliber,it can't hang with Dyneema in any practical sense because the weight becomes unbearable. Dyneema has the best weight-to-performance ratio of any material in the world.

As for heat, that should be a complete non-factor because by the time heat begins to affect Dyneema (e.g., around 240F), a person would be roasted.