I'm actually going to answer you last question first. The main difference is where in the chain the disinterest appears. If someone doesn't have any or many sexual impulses, it means they don't have those moments of going "huh, sex with this person/sex in general sounds fun right now." But someone else could have those desires for sex, but when they think about actually pursuing it, it doesn't sound interesting or feel like something they want. Sometimes, the fact that they never feel like acting on those impulses can be a sign that someone is ace or demi.
That being said, I think not wanting sex unless there's an emotional connection can fall under those other reasons you listed for why people might desire sex but not act on it. For some people, that need for deep emotional connection before sex is an option is a sign they might be demi. But I think lots of people who aren't demi still want some kind of emotional connection before they have sex with someone, even if it's casual or one-time sex. It's very rare that casual sex is completely devoid of emotion; trust, comfort, or general feeling of being friendly towards each other are all things that people factor in when deciding to have sex, casual or otherwise.
Does any of that help?