Passing data from Django to C++ application and back
Well you have to use some IPC method. One that you don't mention here is having the C++ process listen to a socket. That would add in flexibility (with slight speed cost) that the processes don't even need to be on the same machine. 
I've been doing a sort of similar thing, coming from C++ but wanting to write UX in python. My computational backend is C++, and I compile a python module and generate html with flask for the UX. My C++ and python live in the same process so I haven't addressed your core question in practice yet.
One piece of advice I would give is to keep all of your IPC stuff in C++, and write a small python module in C++ using Boost.Python. This will let the python process doing 95% of the work in a pythony world, but give you the bit-level confidence I would want as a C++ dev for the data you are sending over to C++. Boost.Python has made bridging C++ and python web frameworks a breeze for me.
We are creating a trading application, where the backend is totally in C++ (using QuickFix engine). We would like to build a web application in Django on top of this backend, where the user can place his orders. Both the Django (python) and the C++ application will be running in their own processes and address space. What do you think would be the best idea to pass orders/messages from Django to C++? 
Also, this is a trading application, so latency is the biggest concern. So, I do not want to put orders into a database from Django and then fetch from C++ application.
I'm currently looking at doing it via shared memory or some other IPC mechanism. Is this a good idea?