Underexposure was my initial reaction. As for half the roll being fine and the other not, my guess would be that you inadvertently changed some setting on the camera, but then I looked up the camera and it has no settings to change, and the metering system on that camera is supposed to be pretty good. So why was half the roll well exposed and not the other half?
I cannot say for sure. You didn't post pictures from the first half that you feel came out well, so there's no way to compare. I see two areas where there might be problems. First, you are probably using depleted chemicals. Your kit uses blix instead of separate bleach and fix, which is not optimal and is especially problematic when depleted. The greenish tint can be caused by a temperature problem with the developer, but more likely depleted chemicals. Second, the scanning seems to be responsible for a big part of the problem. The negatives you posted are capable of producing much better images. (See attached.). You did not say how you were scanning your negatives.
To troubleshoot, you need to run a roll through with the camera on a tripod in good lighting, preferably not direct sunlight. Send the roll to a good lab or at least process in fresh chemicals. If the pictures come out well, as I suspect they will, then you know the problem was not the camera and was probably the processing and scanning. I doubt the film was the problem, as 2 months in the camera is not an issue.