Welcome to the forum.
RH allows you to use any other editor you want but at the
expense of losing some of its features. Dropdowns are lost. The
simple method of creating links is lost. I would avoid using Word
as it creates problems with the HTML. By all means use Word as your
editor, assuming you want all sorts of problems to deal with when
you put the webhelp on the server.
You can create the whole help system using an external editor
but like I said, you lose the ease of use.
If your last paragraph means can you open a project from an
earlier version into a later version, yes you can. Or are you
wanting to start from the CHM itself? If so see the Reverse
Engineering topic on my site.
You've put in a link for people to email you but we tend to
discourage that except where a behind the scenes exchange is needed
to sort out some issues or exchange files. Simply because it
prevents others from seeing the answer and learning from it.
Help others by clicking Correct Answer if the question is answered. Found the answer elsewhere? Share it here. "Upvote" is for useful posts.