Yeah, the spinner is used mostly at the start to dislodge any bubbles, but I find that one drop of photoflo in the tank does that better. Just stick with the inversions. If you're spinning, I think you need to spin really hard to get sufficient fresh developer to the middle parts of the roll - just think of the physics of it, there's nothing forcing the exhausted developer out of the middle. With inversions, you have air bubbles rushing through the whole roll, turning the developer over completely.
These dark blotches, are they in the silver layer or on the film surface? If the latter and they look like waterdrops, they could be drying marks. If they're denser regions in the silver, it's likely you have poorly-mixed developer but I've never seen that with Rodinal, only powder developers like D76.