As for knowing the voting weights of the other ~17%, we can't, unless they tell us. All we can do is count up the votes on the blockchain and see what is actually being voted for.
Sure we can. We can count up all the votes from the blockchain and then substract what JD reports. That tells us exactly what the rest (non-JD) of the blockchain is voting.
The only way this is less accurate than what JD is reporting is the natural variance of who gets to stake a block. Over a week this should be reasonably close to overall voting preferences.