Let me see if I can help with some of your questions
1. Yes, you can use LCDS for simple forms processing. Any time you want to
move data between the Flex client and the server, LCDS (or its free Open
source cousin BlazeDS) is going to help. I would expect you would use the
mx:RemoteObject MXML tag to invoke server side code, passing it the form
data input by the application user.
2. If you need to push near real-time data, LCDS gives you the RTMP channel
which can scale quite nicely. You can then use the mx:Consumer MXML tag to
subscribe the clients to the messages, which can come from almost anywhere,
include JMS topics or queues.
3. RTMP (included in LCDS) is the best option for scaling to tens of
thousands of users and the LCDS servers can be clustered to proved better
4. The AMF3 protocol used over the RTMP channels performs much faster than
simple XML over HTTP. See this blog posting for some tests: http://www.jamesward.org/census/
5. If you are sending a Flex application XML, then I would recommend using
the E4X API to work with the XML. This is a pretty nice and powerful way to
work with XML. If you want Actionscript objects (and probably better
performance), then using AMF serialization to Actionscript objects is the
way to go.
6. Primary advantages? There are many, but mainly you can avoid thinking
about the plumbing and concentrate on solving your application and business
Hope this helps you a little