You can probably ignore the cherry tree droppings. They aren't likely to be thick enough to matter. Give your lettuce 2 weeks before you give up on them. It's possible to drown lettuce, but it's hard, so her advice on watering is good. I would have watered mine today, only the sky did it for me.

Next year, you can start the lettuce before the last frost, so late April/early May? Lettuce can take the cold and you'll get lettuce that much sooner. You can also start lettuce in September, for fall eating.