The problem is in the way the dates work. You have to book 180 days from the arrival date, not the departure date since the trip spans two dates with the overnight. As for the fare going up, I found in booking for our family of four that I had the same problem. What happens is they only offer one or two tickets at that deeply discounted spar-night price, then they go up from there. If you try to book more than the number available, it puts you to the next fare level. The way around this is to book one or two for the super cheap price, and the remaining at the higher price. It involves making two bookings, but otherwise you just have to pay the higher price.
For our family, we decided that when the price went from $384 one day to $730 the next day when the tickets could be booked, we went to the sleeperettes, or reclining chairs, for a total cost of $185. All four of got the cheapest price, and when I enquired again the price had gone up $128, so there were only 4 such seats.