You can link (not import) each individual document in RoboHelp. Whenever one of the documents changes, you can simply regenerate the topics.
I don't believe sub-docs are supported by RH and I doubt they will ever be. Sub-docs are a fool proof way of corrupting Word documents, but perhaps MS changed that in the 2013 version. Personally, I would always advise against sub-docs. If you need this kind of flexibility, I would recommend taking a look at FrameMaker.
Will the "RH linkage" also consider index and toc entries? I never tried before.
If you write >800 pages of documentation, sub-docs are the only way to keep things fast and stabe in word. I use this sub-doc procedure since >15 years with different authors and translation
You can convert the index and TOC as well. It's a bit more hidden because you first link the document and then choose to convert the index/toc.
I know the pain of long documents in Word. If you have no problems with subdocuments, you're either extremely good with Word or extremely lucky! I hope for the first. Personally, I live by the advice given here: http://www.addbalance.com/word/masterdocuments.htm
I gave it a test and it does more or less what I need. But: why are the HTML output files using "WD_" as prefix to all CSS formats I defined in the .ISF file? How can this be turned off, since when I manually import a word file, it does not happen. I hope I do not have to write another .JSX again..