a. A category is unchecked if it's excluded on the Advanced options panel. It's the best case use, since you can give the user the option to include the posts from a certain category, even if it's excluded.
b. Going to happen eventually, perhaps in the next update. Checkboxes and dropdowns work very differently on server perspective, and the current state of code is made exclusively on check-box versions.
c. I'm not sure about this.
Grouping to dropdowns based on parent categories – I'm not sure if this is going to be a feature, because of taxonomies. To define a group, wordpress offers using taxonomies, which is well supported core feature. In your case “Year”, “Genre” should be the taxonomies containing the terms (categories).Best,
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