If I were to do that I would lose all the work I'd done to manually set all those fonts and custom line spacing.

I've learned my lesson and in the future, now that I understand more about styles, I'll use Normal style for the English paras, and try and set up a custom style for all the Hindi paras. That way everything will be standardized and systematized.

But for this current book it would be too much to lose all the time and work put into setting up the fonts etc. Could there be a workaround of the sort I described above wherein I create a new custom style called "new" having the attributes of file 3 i.e. 12 pt Times Sanskrit, and could I bring that style into effect at the very site at the end of 'Part 1' where I've created a new page section break for merger of file 3 into 'Part 1'?