How to prioritize subtitles in LAV Splitter?
I've managed to ensure the subtitles prioritise forced, then to choose non-forced English as a backup using this:

*|f,eng*
However, this does not guarantee it will always choose forced English (assuming this is your preferred language). So to ensure that you will always get English, I have found this seemingly works consistently

eng|f,eng*,*|f
This checks for forced subtitles in English, if not found, it checks for non-forced English subtitles, if neither are found, it chooses the forced subtitles.
As for setting the default audio track to Japanese, I found adjusting the order to preference did the trick.

NOTE: This is not guaranteed to work for every file, there are many variations to the way subtitles are setup. I have just found this way to be the most consistent.
If you want to learn what these symbols mean I suggest studying Regular Expressions or Formal Languages, or straight up do an online course on Theory of Computing.