Hi,
Go to Tools-->Form Options, In the General tab,there is a checkbox to enable merging.Tick that checkbox.
In the File menu,there is an option to merge.
Note however that only the repeating group data gets merged and not the rest.
For ur 3rd doubt, u need to preview the form to see the changes made in ur form.Click the preview button.
and I dint understand ur first doubt.
Regards,
Abhishek