During the exposure, the reflex mirror (the 'R' in dSLR) is flipped up and completely blocks light entering the viewfinder from reaching the sensor. If your camera is leaking light during long exposures, you should send it to Canon Service, because light leakage sufficient to affect an image directly means a defective camera.
No, not quite. Normally the mirror blocks enough light from the viewfinder, so that it is not seen in the picture. But if you use a filter like ND 3.0 or IR 720, which lets only 1/1000 of the light pass, the light from behind can be clearly seen in the pic, even with a non-defect-cam (on an IR-image it looks blue).
A lamp from behind during a long exposure can have a similar effect.
And btw. a viewfinder-cover is also usefull, when you do timelapse with exposure-metering...