Yes, with coal the operational cost varies with usage, while with nuclear it doesn't as much, so it's less flexible expense-wise.

About exploitative margins, the industries I mentioned have high margins because they don't have to pay for the externalities directly related to their activity, or very little. If you increase the costs through regulation, either the consumer price has to increase, or the margin has to shrink. Increasing the price is a gamble, as it can reduce usage and overall profits despite a similar margin. Reducing the margin is safer to sustain the same level of activity but also makes it less attractive for financial speculators.

I'm not sure what is the purpose of "Big Energy" in your last sentence. Energy as a public utility needs to be centralized by definition.