Welcome to the forum.
With questions that diverse it might have been better to post
them separately so that people with knowledge of some of the
different elements can reply. Good job it's Sunday and I have the
time to tackle them all.
RH Server is not RH Office. RH7 gives you RH HTML, RH for
Word and RoboSourceControl, and other bits. RH Server is only
required if you want to use the WebHelp Pro or FlashHelp Pro
outputs. There's information on Adobe's site about what RH Server
gives you extra. Essentially user feedback and reports.
There's information on my site about JavaHelp, perhaps that
will make it clearer.
Single source surely implies multiple output. A single source
for multiple outputs. In one project you can have many topics which
could cover, say, different products. In any one output you can use
whichever topics you want. So you might have 10 topics about your
company and 10 for each product. If you want an output that covers
all the products, you use all the topics. If you want an output
that covers just some, you use the core 10 and then the 10 for each
product to be included. Thus you have a single source of topics and
multiple outputs.
With DITA, you have maps to say include this topic. With RH,
for online outputs you use conditional tags to exclude topics thus
defining what does get included. For printed outputs, you can
either exclude in the same way or define what you want included.
Help others by clicking Correct Answer if the question is answered. Found the answer elsewhere? Share it here. "Upvote" is for useful posts.