Unfortunately I get mixed info on aspect ratios depending on where I look, and I don't know how to interpret the confusion, and I don't see any source being more authoritative than another. The US theatrical ratio was definitely 1.85:1, so maybe the sites reporting that as the OAR are just being US-centric. But then the aspect ratio on the film itself was definitely 1.66:1, so maybe the sites reporting that as the OAR are just using "original" to mean "on the film" rather than "in the theatre".

Hard to tell. It's also common practice to soft-matte a film to 1.85:1 or 2.35:1, but then when the film is released on disc, to open the mattes back up to whatever was on the negative, resulting in a taller aspect ratio. Frankly given my cynical take on Blu-Ray mastering (and the fact that other MP films are 1.85:1, why would this one have been presented differently?), I am still inclined to believe this is what happened with the Holy Grail. I certainly could be wrong though.