I stand by my opinion that the rules are in conflict, even though I agree that there is no mathematical obstacle to applying them both simultaneously. Instead, the conflict I see is in the purpose of the rules.
Specifically, I read the grappling rules as designed to enable one creature to drag another, not to further limit dragging beyond the speed restrictions on dragging inert objects. Far from enabling dragging, by applying both movement limitations simultaneously, one average human (carry capacity 150 lbs., weight 165 lbs., 30' speed) could only drag another average human 5' per round, and that only if they take the Dash action. That the average case would fail to allow practically useful or even vaguely realistic dragging speeds I see as further evidence that the rules are in conflict. If dragging any meaningful distance was indeed restricted only to strong characters dragging lighter creatures, I think the grappling rules would have been written in a way to make that additional limitation clear, rather than presenting the grappling rules as limited only by creature size and relying on readers to infer that the general weight limitations for dragging still apply.
I'm all for stopping Spirit Guardians shenanigans, but doing so by interpreting the rules in a way that nerfs dragging into practical uselessness I don't see as a net improvement.