Hi Ahmar,
First of all, I'd like to kindly ask you to keep your related questions on the same thread here, just to avoid duplicate questions.
Your use case looks like a very straight forward GemFire use case. On this link you can find a more detailed description about some of the quickstart examples that come bundled with product - vFabric Documentation Center - Including partitioned and replicated regions. The examples include Java code and many cache.xml files (including client/server files) that can be used as a base for your case.
About your questions:
1 - Well, the common answer here will be "it depends" - But I do recommend for most of the time to keep your region configuration on the configuration file (xml) so you can modify your region properties without necessary changing or compiling code. The Java API would be more indicated for very dynamic cases where regions needs to be created on the fly with some characteristics based on a runtime information.
2 - On the documentation there is a description of each region type and a column with recommended usage for each type. Here is the link vFabric Documentation Center
3 and 4 can be addressed after reading the Gemfire best practices guide which I'm also giving you the link here -http://www.vmware.com/files/pdf/techpaper/vmw-vfabric-gemFire-best-practices-guide.pdf - The document provide lot's of helpful tips and recommendations about your JVM and Gemfire settings. There is also a GemFire Memory sizing guideline - GemFire Memory Sizing Guidelines
5 - Please check my 1st paragraph. There are examples included in the quickstart folder of the product. For client and server configurations.
6 - Not sure if I understood the question, but yes, we have API's for retrieving data from the regions: "as you put(), you get()" - 
Hope that helps and looking forward for you/your company be successful on your POC.