Yes.  Using appropriated funds to pay honoraria is authorized as a necessary expense for invited guest speakers. 
 

There is no set amount that is authorized, but the general rule is that the amount must be reasonable.
 

No.  Honoraria are only authorized to the extent they are necessary expenses for invited guest speakers.  A conference where everybody participates by definition has no featured speakers.  To the extent that a few attendees are asked to speak for a substantial period of time or to serve together on a panel on a particular topic, those few attendees could be offered an honorarium if it is determined that the honorarium is necessary for the accomplishment of the conference.