Kitty McGeever - the actress who plays Lizzie - is blind in real life so she cant really all of a sudden not be blind in the show!
Just because she is blind, doesnt mean she is completely useless!!! I hope that is what the show is trying to portray. When someone loses one of their senses, the others become extra sensitive so with regards to knowing where to throw the eggs - her hearing would be alot better than the average persons - meaning she could hear exactly where Eric and Val were without seeing them.
Also, we dont know as yet how long the character has been blind for - could have been since birth meaning she may have adapted certain things that most people take for granted. Her writing in a straight line, for example. She may have developed a technique over time so she knows how to write straight. She lives a normal life - except she cannot see.
As for finding her way around the village......she is blind, not stupid. She has obviousy got her bearings and has adapted a method to do this.
If she needed constant help to live her life, it would make it seem as if being blind made you completely incapable of doing anything for yourself!!!