Does "besides" mean same as "except for?"

Does the word "universe" in this context refer to "parallel worlds" instead of "space?"

Close! "Universe" in this context is similar to "world" or "reality". So then the phrase "every universe" implies "parallel worlds" - that there are multiple "realities" or "worlds." As "every" means all individual units. Ex: Every business in the country is closed for the holidays. Ex: If I had to travel to every universe to find you, I would.

What's she mean by "ass" here?

Here, "ass" means a foolish or stupid person. It is considered rude and slightly offensive to call someone an "ass," so it's good to consider when you use it! Ex: He's such an ass. I can't believe he was so rude to you. Ex: Why are you so nice to such an ass?!

Is "by" the usual preposition that goes with "mistake?" What's is mean?

Yes! "By" is a usual preposition paired with "mistake." It means the way something happened through a mistake, by way of, or because of a mistake. It was not done purposefully or with intention. Ex: I broke the vase by mistake. Ex: So, by mistake, I slept in and missed my test.

What's he mean by "treacles?"

"Treacle" is a type of thick, dark syrup made of sugar. It's sometimes used to describe excessive flattery or sentimentality. Because it is a type of "sweet" (both metaphorically and literally), my guess is that the speaker uses "treacles" as a pet name for his audience, something similar to "sweeties." However, this is British English and not necessarily commonly used this way. Ex: Sweeties, I have a surprise for you. Ex: I like to add treacle to my batter when I bake.

What's she mean by "Idol" here?

"Idol" here is short for American Idol, which is a very popular singing show in the US (and other countries). Every season, amateur (and sometimes professional) singers compete to win first place and hopefully get a chance to become a big star. Ex: My favorite singing show is American Idol. Ex: Who's your favorite Idol winner?