The old InDesign to Frameamker conversion trick....

While you CAN import word files into FM and have FM map the Word styles to FM styles, there are a lot of other stuff that does not work very well. Tables, Cross references and special formatting like super/subscript etc.  If you are thinking of setting up a round tripping environment, you will need to do a lot of testing.

And (dare I hope) nester styles?

The only nested style that FrameMaker supports is assigning a character style to an auto-number. So no, unfortunately that's not going to happen.

~Barb

InDesign doesn't save the file as RTF, but can save text flows to RTF. So, depending on how the document was created, it could be simple or extremely difficult. 

If you want the entire file converted, this is an old MIF-to-IDML service that might still be available:

https://dtptools.com/product.asp?id=mfid

You will have to contact the developer.

Returns, tabs, and most features would be supported in the RTF and into InDesign.