This does look like it could be a light leak. Since it is in the same position each frame, the leak is letting light onto the film as it sits ready for the next exposure. Since the leak seems to be coming in at the top of the picture a little right of center, the leak is at the bottom of the camera, a little left of center of the image rectangle.

How much the leak shows up in a picture depends on how long the film was sitting in position before the picture was taken, and how bright it was around the bottom of the camera. Assuming you wind the film immediately after taking a picture, the amount of leaked light in a particular picture has to do with how long it was taken after the previous picture.

A good way to verify this is to look at the film directly. If what I'm saying is correct, then the light from the leak will be evident at the sprocket holes area outside the image area.

Added in response to updated question

This is now definitely a light leak in the back of the camera, as evidenced by the light streak not stopping at the edge of the frame area. Keep in mind that the images at the back of the camera projected onto the film are upside down. The main light leak is therefore at the bottom of the camera a little left of the center of the picture.

The reason I say the leak is at the image area as apposed to one of the spools is because the streak is in the same place each frame. Even if there is exactly one frame per revolution of a spool at a particular picture, that would change as the diameter changes due to more or less film wound around the spool. If the leak spacing was regular with regularly changing distance between leaks, then it would indicate a light leak at a spool.

The light where the cartridge goes is not all that useful in this case because it wouldn't make it over to where the leak is at bottom middle of the camera. After all, the light entering there only hits a thin vertical section of film, so light coming from elsewhere wouldn't make it out the leak hole.

Look very carefully at the light baffles where I said. There may be a nick in one of the baffles there. The fit may also not be as tight as it should be. Close the back, then press on it a little and see if it can be pushed a little more closed. If it can, then it's not being held closed properly.