Thank you so much for that suggestion, but it does NOT work.
I have tried converting the .rtf file to a Word .doc, then to a webpage-filtered, and then importing it into Calibre, but I experience the same exact problem:
Calibre first inserts spaces between paragraphs and removes the tabs; then it reverses itself, deleting the spaces and re-inserts tabs. However Cailbre is "dumb" and sticks tabs after every paragraph mark. As a result, all centered text is pushed slightly to the right, and it's impossible to have any non-indented sections.
In MS Word I tried replacing the paragraph marks with manual line breaks before the stuff I wanted Calibre to ignore when inserting tabs, but that did not work. Calibre also forced tabs after manual line breaks.
Again, this problem happens regardless of whether I directly import the .rtf file or first convert it to webpage-filtered. I've experienced this problem for years, so it's happen with multiple versions of Calibre.