please keep your “darklight theme” in the Darklight (Beta) release. This keeps the forums clean. The best person to answer theme questions is the theme developer and they subscribe to their own threads.
First, as I said before, if you have a question about a theme, please post it in that THEMES THREAD. There is no reason to keep starting new threads if you have a question about a theme. The more threads you start the harder it is to navigate the forum for other users. If you have a question regarding Darklight, then ask it HERE, I will answer it when I see it.
But to answer your questions
Yes, the only one that you can delete is the “filename_cover.jpg”, but I don’t recommend it since this is a backup for your coverart and can make it easier to regenerate the Coverart if you have too.
Yes, by modifying the template, but I don’t recommend it due to some covers will not be able to be seen very well if you make them transparent.
Yes, this a firmware limitation
The easiest way to do it (and this is just from memory) is to go to Options, then set the Extra sheet for “$M\folder” and output as “.jpg”. When you generate the moviesheet for the first episode then select the check box next to Extra sheet, then deselect it for the rest. There is another way, I just can’t remember from the top of my head on how to do it.