Why is the blank tile not always the bottom right one?
When Domi-X generates a new puzzle, it makes sure that it can be solved - sometimes it can't be solved if the bottom right tile is missing
How can I maximize me score?
Use undo if you get into a "dead end" - your score is based on how many moves it takes, so use undo rather than manually moving the same tiles twice. Similarlly, if you are taking a whole lot longer than the number of steps the perfect solution has, restart the puzzle and take another strategy. It is also always better to finish the puzzle, regardless of how many moves it takes, than to give up.