If your exposure outside was f8 at 1/50 sec, with PROVIA which has an ISO of 100, cloudy bright conditions would indicate an exposure of f8 and a fraction @ 1/125. So you by all indications were about a stop off and with the polarizer would have been a stop UNDER exposed which would have given you DARKER images not washed completely out images. It is a puzzle, and could be one of several things....Light leaks, defective lens/shutter, etc. But the fact that the pinhole images were equally bad might lead one to suspect bad film processing. I'm not saying your exposure computations were spot on, but they were not off enough (on the shots made with the lens) to give you completely washed out images-IF all your equipment was working properly.

For 4x5 color transparencies, I always shoot a Fuji Instant print and evaluate it, because I hate to bracket with expensive transparency film.