All witnesses from current schedule are taken into account, which means top 20 plus one backup that happened to be in schedule active at time of feed entry creation.
Thanks, these kinds of details are sometimes hard to find.
The rest of your comment happens to be rant from the second sentence on. I agree with you that HIVE/USD price is different to two randomly selected Hive users. That is the exact reason why the blockchain asks the question - if a single price existed, noone would need oracles. And of course, witnesses can use all kinds of info to make their educated guess.
An extreme example to try and prove your first sentence false: If Witness X broadcasts their Social Security Number by mistake, they have just broadcasted HIVE/USD price. A totally dumb one (that won't influence the Hive price feed fortunately) but still a HIVE/USD price by definition.
They cannot broadcast HIVE/HBD price. They can make a Hive post about it but they cannot broadcast it as there is no such variable in the questionnare they got from the chain.