Hi Rajesh,
I have very little experience with in client addons, so I will try to help as much as I can, but your best option is probably to post separate questions for each issue as you encounter it.
As far as I know you can declare events "handled" the same way as in normal windows applications. That might be the answer to all four questions.
Another approach could be to process the whole order at the end, when pressing the ok button. In other words, just let the system add rows the normal way, and when the user approves the document you first combine all "double" rows, perhaps with a second approval question, before letting the system actually create the document.
Regards,
Johan