Ok, here's a breakdown for you:
1) You need to read the license file that was included with RSMB when you downloaded it. My understanding is that you can't use RSMB for commercial purposes and if IBM release it as a commercial product you have to stop using it.
There is an open source MQTT broker available called Mosquitto (which I develop). It provides full MQTT protocol support and is available for a number of platforms. It also includes client libraries in C, with C++ and Python wrappers, as well as example publish and subscribe clients. You can get more information at http://mosquitto.org/
2) There are two C# libraries available, although I've never used either of them myself. You can find a link to them as well as a whole load of other MQTT related software at http://mqtt.org/software
3) I've had RSMB and Mosquitto handling 1500 messages (100 bytes each) per second on an old VIA C3 600MHz machine. On more modern hardware I'd expect them to perform significantly better.
With regards to information on MQTT, I hope that there is a reasonable amount on the mosquitto website - see http://mosquitto.org/man/mqtt-7.html for an overview for example. You can also come ask questions on the #mqtt irc channel on the Freenode network at http://webchat.freenode.net/ There is also a mailing list at https://launchpad.net/~mqtt-users which may be of use.
I'm afraid I can't really comment on whether you should be using MQTT or something else.
