I can’t really answer most of your questions, though @mike did ask about the utility of perhaps including Reverso Context sentences here in Clozemaster, which @morbrorper did echo would greatly benefit languages with fewer Tatoeba sentences.
Fluency Fast track, in my understanding, does normally start with the most common words (which you could all now just mark as “known” at least) to then progress to the less common words. In Italian, and other languages, we also have the Most Common [100 / 500 / 1000 / … / >50,000] Words Collections, where you could just jump in where you’d like. I was also beyond beginner level when arriving at Clozemaster, but still found it to be very beneficial, since, even if the first few clozes are perhaps relatively easy, I was still learning a lot just from seeing them in the context of the rest of the sentence. It’s not just about translating / learning the cloze word for me, but about the cloze in the context of the overall sentence, so I will still learn a lot, even if I already know the clozes in question.
In my experience, you will see a lot of the same sentences again later on indeed, with the “less common” clozes now selected, instead of the previously “most common” cloze words, which is basically what you’re fast-tracking by adding them with different clozes to your own collections. I think you can safely mark the original sentence as “known”, and sitll be confronted with the sentence with a “less common” cloze later on, since the “known” marking should occur for that specific cloze-iteration of the sentence in my understanding.
Anyway, most of these things are just speculation from my part, I mainly wanted to show a sample csv file.
You can probably most easily make them using Excel (or a Google or Open Source or other equivalent), you just need to put the “Sentence” in the first column, which is the only required column for adding sentences yourself, but it’s generally also helpful to put the “Translation” in the second column, and select the “Cloze” word of your choice in the third column. I’ve never used/included the “Pronunciation” or “Notes” columns myself, but they would be the fourth and fifth columns. You can leave the Pronunciation column blank, and still add a Note, but it would have to be in the fifth column, in order to get read in correctly. Here’s a screenshot for reference of what that would look like:
Also note that I’ve added the same sentence twice here, but with different cloze words to be selected, as the first and second entries.
From Excel (or equivalent) you can then save this file as a .csv file, which is a “comma separated file”. If you were to open this file again in Excel, it will still show you the contents in the columns, as expected, but if you were to open it with a text editor, you would see the content of the “columns” separated by commas instead.
Note that the “Notes” column, contained a comman for the second (and only) entry, and it put quotation marks around it, otherwise it would think there was another sixth (undeclared) column with " just for illustratory purposes" as its content for that row. This is something to pay attention to if you’re not using Excel (or equivalent) to generate the csv file, but rather trying to create it yourself manually.
If any of this is still unclear, or you’re running into any errors creating or adding the csv file to a collection, just give a shout and I’d be happy to try to help troubleshoot and/or try to explain things more clearly 
