Yes I use all default fonts and emoji's. For the font symbols as button symbols I think I need to use something like font-awesome, I'll add it to the todo list!
I think for the first levels that is true, but levels around 300+ will take quite some time, and then the time between play and the transitions is minimal. But good feedback, I will see it such a thing could be possible. It would help in the early levels.
For now the level number you get to is sort of your score, There are a lot of levels, and some players are already around level 400 :-) (no analytics, just from my social circle) being able to scroll back to get a level from 2 to 3 stars does not seem like a good effort somehow. But I will think if there is some way to measure differences in play, if that is really interesting to know.