http://iscoresports.com/

I have only used iscore on a phone, not a tablet, so not 100% sure this doesn't already exist...
For the Undo , show the last few entries/plays on a stack and undo would then just erase the last entry , but you could see where to stop for example, if undoing a few steps.

Undo does undo as far as you want. On an iPhone, we do not have a pitch by pitch listing on the scoring screen, but on tablets, you see each play and can see which one is being undone. This just has to do with screenspace.