Thanks for reading this article. The first question which you have is really a tricky one. I may not be correct on commenting in respect to this question because I did not find any dependable resource, but for my server, it takes 80-150mb approx for the kernel to run. However, you can adjust the MIN SERVER MEMORY and MAX SERVER MEMORY and see how much memory allocation will be optimum for you requirement. Once again, this comment is solely based on my observation and may be different than actual.
For the second question, every connection will take 64bytes of memory. It is not wrong, to assume that 2 instances connected to the same server and database will consume 2x memory! Actually, if you have n instances open for any server, the server will reserver 64n bytes of memory. Now, this brings us to the important consideration for the connection memory utilised and we do have a counter for that too in our favourite perfmon! :-)
The answer for the third qustion is to liverage your operating system architecture. In Windows operating system, we can run an application as a foreground process or a background process. If we have an application which is not accessed by foreground, then it will be convenient for the OS and the App to run in background. If you want to run any foreground process on that server, other that SQL apps, the OS will be more efficient on managing the memory. Once again, I am not in the place to comment on the actual benefit it would cause, but can certainly say that it would be apt for the process and you would be getting the benefits in certain scenario as discussed above.
Hope this clarifies some part of your doubts. Please do write if you have more questions.
~Arindam (not Ravi) :-)