A much better solution to monitoring traffic on a per user basis would be Traffic Flows, or Netflows.
http://wiki.mikrotik.com/wiki/Manual:IP/Traffic_Flow
Using simple queues for that really isn't ideal as they are not designed for it, and it will eat up extra resources on the board to have them running. Also, even though you can specify to have the graphs saved to the flash disk, I have had graphs disappear upon reboots very often even with that setting set, so storing them on the router is not ideal.
But to answer your questions. Simple queues are that, just simple. They really don't have many options or controls with them. I'm not sure why specifying an interface didn't work for you, but simple queues are attached to the Global-x part of the packet flow, and I'm guessing it has something to do with that. By specifying the interface I'm guessing you are only getting 1/2 of the connection (I'm assuming you are specifying the LAN as the interface), without knowing more of your setup I cannot guess any further.
The reason why you are seeing Tx and Rx as backwards is because everything is from the perspective of the router, not from the client machine. So the router sending a packet to a client is a download for the client, and a packet being received from the router is an upload.
If you are just looking for traffic monitoring, then no you don't need to understand queue types. Queues in and of themselves are designed for QoS setups, so if you want to do any of that, then you will need to understand them to a certain degree.