Go back to my crude definition of a niche: "A set of obstacles that separate an organism from nutrients and energy it requires". By this definition, niches may definitely exist independent of if there is an organism at any given time that has the necessary tools to overcome these obstacles, yes. You will have to provide evidence that this is something evos deny, as that is such a self-evident statement that I will otherwise not believe you. If ID claims this as well, that still does not prove the existence of God.
However, it is true only for some niches. The set of obstacles that form a niche are only partially constant over time. Other obstacles, which are dependent on the structure of temporarily limited features are not constant over time, and may very well be lost in a major, or even minor, extinction. It may also be at least potentially possible to create your own niche.
Not sure what you are talking about here. It's pretty simple. The environment for an organism includes the living biota around it. When you wipe out biota, you change the niches.
From what you wrote, it's not clear whether you think niches exist as potentials prior to being filled, or are simply created when they are filled.
Which is it?
Be specific. Was the apex large ocean predator whether dinosaur or whales, a niche that existed waiting to be filled?
If not, why would you say whales refilled that niche and that dinos going extinct opened up room in that niche? When the dinosaurs went extinct, there was no niche there, right? Whales had not created that niche, right?
This really is something worth thinking on. I will await your response despite the needless smears and so forth.