I updated .inf and converted blend and water masks to .tif with GIMP (no transparency). Yes, I always saved my .bmp files in 24 bits exactly as mentioned in the tutorial. However, my BGL compiles normally without any error. The problem is that my scenery worked with such settings (bmp, tif) before I add seasons, so it worked only when I had ALL variations on the same Imagery.I didn't see the April duplicate, but it still compiled for me using 8-bit greyscale tifs for the masks. (It shows mild-winter for me in TMFViewer).
In your original INF file you are using a blendmask tif and a watermask bmp. Maybe that was the original problem? As far as GIMP... I don't know what is being saved, but I recommended 8-bit greyscale TIF. You could zip the images or if too big put them on a One drive or some other source for us to download and look at them.
Resample stops if you use a bitmap with 8-bits. Bitmaps need to be 24-bit (I think 32-bit might also work). It's hard to get a greyscale with a 24-bit palette.
My compiled BGL is about 2.41 GB. Is this normal?