1. What is the difference between a roleplay and a round robin?
By 'Round Robin' I think you mean a collaborative writing activity being executed by people in turn or in a specific order? If that's the case, roleplaying here is more free-form. People can post at any time, though it's usually considered poor form for you and one or two other people to 'machinegun post' and just do loads of back and forth without other people getting a written word in edgewise.
2. How often should you check your roleplay?
That depends on the size of the cast and the sort of place it's set in. If it's, say, a Kingdom Hearts roleplay, people generally are more confident to act without supervision and so the manager's attention wouldn't as often be required. In an original roleplay, where things might move more slowly, people can sometimes require
more manager attention than a canon or KH one specifically
because it's in an unfamiliar world. Ultimately, you typically figure out what's best for you through just roleplaying and managing.
3. What do you do if someone stops posting?
If they're still active on the forum but not taking part in the roleplay, warn them that they'll imminently be removed. If they're inactive on the forum too, you should try getting onto them through some non-KHI method such as with emails or instant messaging.
4. How do you properly end a roleplay?
That's completely up to you! Sometimes it occurs when all the loose ends are tied up, or when the final battle has been fought and all the characters are ready to part ways or continue on into the unknown, or something like that. As with the medium as a whole, it's very free-form and open to however you want to do it. If a roleplay just sort of dies, most people generally consider it to be ended, and such a post announcing this in the corresponding OOC/discussion thread would just confirm this.
5. What do you do if you find someone is completely taking over the roleplay?
Speak with them directly, telling them that they're overstepping their bounds or directly doing something that shouldn't/can't be done in the roleplay. Failing that, contact me or Takushi Rena about it (I'm more active than her but she's around every few days) since we're the roleplaying moderators.