Killer Sudoku Challenge 4+

Suggestions to Bring Rating Up

Things that I like, besides that it is killer sudoku:
1. Being able to enter pencil marks.
2. Pencil marks are always displayed in numerical order.
3. Ad at the bottom is unobtrusive.
4. Games are included with the installation. No need to buy them. Although the hints need to be purchased, I never use hints. Since you're not going to make money off me, maybe the way you can do this is to add more puzzles (especially ones more difficult than level 4) but only AFTER you fix all or some of the isues, especially the nonunique solutions one.
Things that ruin it for me - fixing all or some of them would bring the ratings up.
1. THE most important, and the main one that gets this only a single star (and would have been zero stars if permitted) - Solutions are not unique!
2. Level 4 (IV) is still not difficult enough for me. Need a level 5 or higher!
3. I don't like being timed! Need a way to turn off the timer but keep the game going. And remember the timer turned off between sessions! Make it one of the options.
4. Need an undo button plus a quick way to clear pencil marks from cells.
5. It's not made clear what the sum option is. I didn't notice what effect it had when I toggled it on or off.
6. Need a way to check if the whole board that I've entered is correct. Right now, the only way to get any type of checking is to put in a number that's already in the row, column, square, or number cage. Which leads me to
7. I actually don't like seeing the number in red if it's a duplicate. I prefer to enter it and live with the consequences.
8. Personal preference - I like my killer sudoku puzzles to be symmetric. I think I did get one symmetric puzzle, so maybe you mix them up. If there are at least some symmetric ones, that should be okay, since others may prefer ones that aren't symmetric.