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The document I'm going to index is quite big: a total of about 8MB. Because of its size, , when I worked on it, I have found it necessary and practical to split it into five parts. To make an index of names of modern scholars quoted, for instance, I would rather work on a single file, but when I try to combine the five into a single file, the pagination gets disrupted, and to revise it would be a very messy and tiresome manual work, for the text has already been submitted to the publisher and its pagination has been definitively set by the publisher, and my computer file has to work in line with the publisher's. And apparently, because of the incompatibility in their software and mine, they can't provide a file which I can process and produce indices from under NWP.
At this stage, when the publishers have the final pagination of the work, I think the best solution would be to ask them if they can create the indices. If you sent them your NWP rtf file with topics marked for indexing, when your file is imported in their software, say InDesign, the index marking should also be imported as I mentioned here:
Then they only have to run the Generate Index command in InDesign for the index to appear with the correct page locators as they appear in the document in InDesign.
If the software they use cannot import your index marking, you can give them a list of topics (index entries) that you want to appear in the index(es). Publishers use powerful tools to create indices. They only need to know what should appear in which index. Also, their output will be most accurate and save you all the trouble.
Let me know how it goes.