Hi Milind,
To answer your first query, in order to avoid getting the pop-up on the entire screen, what you can do is to pass an initial default value to the variable from the "on Variable Initialization" event in the Application Properties. You can do this using the following syntax:
APPLICATION.setVariableValueExt("<Technical name of the variable>","<Some default value>");
This should prevent the default prompt selection screen from popping up on start-up. Post this, you can go ahead giving the command to open the prompt dialog inside the "on Startup" event under Application Properties.
This technique will not require the data source to be loaded in script. If you would still like to load the data source in script, I would suggest that you use the "loadDataSource()" method along with the"setVariableValueExt()" method mentioned earlier.
I'm still looking into your second question. Please do let me know if this works.