1. I see no reason why one shouldn't post the quiz as soon as the previous one is finished. Of course, you'll have to make your own judgement whether players are "fed up", and whether you should wait or not.
2. Of course, it's usually not a smart move to repeat "themes", but if you think you can say something new on the specific theme there're no restraints. Me myself have often found more pics of the theme i've already done, and wished to do "sequel". After all, ask Trunkyo - his "Trees" quizzes are popular series.
3. That's exactly why there's limit of (usually) 2 guesses, and rounds - so everyone, or almost everyone should be given an equal chance to participate. There're also time zones to consider. But everyone is free to make his own rules, and there's always room for improvements.
4. I don't like PMs. It can work of course, but for these type of quizzes i like to follow as the quiz "moves", regardless if i'm seeing only white spoiler lines, or actually reading them, like when i'm almost certain i can't guess it by myself.
