If I reverse mount a Canon 50mm F1.4 prime lens on Canon Rebel T3i/600d can I still use manual focus for macro?
We have a great blog post here at our own photography stack exchange blog that explains the process very thoroughly: Take macro shots like this for less than the cost of a pizza
To answer your question, yes you can still manual focus, that is the only option as autofocus will not work when reverse mounted. Manual focus is a bit odd feeling, because the mount is so small and doesn't really give a solid "mount" feeling as a normal mount would.
You don't directly have control over aperture, but you do on a Canon body by using the depth of field preview button(Using tack on a Nikon lens). The blog post listed above has the details on how to achieve this specifically, but it is a very nice "hack" indeed.
NO. Manual focus will NOT be available with a Canon EF 50mm f/1.4 that is reverse mounted to the camera body.
Reverse mounting attaches the filter threads, and all the glass elements of the lens, directly to the camera body. When you turn the focus ring the body of the lens will move back and forth but it will not move the glass elements.
The only way to achieve focus is to move the camera forward and backward.