It looks like you already have a script that does most of what you want. You're already getting the offset of the line.

For the first question regarding offsets, what have you tried, and why didn't it work? Your script stops short of making any effort to actually do this. This isn't a place where you can write half a script and then expect us to finish it, so what specifically is stopping you from accomplishing this?

As to optimization, never do it before you've done profiling. If your script is not running acceptably fast, find out where the slow parts are and address them. You need to do profiling to identify the slow parts. Do you need direction as to how to perform profiling?