There is a simple solution SE could implement...
Force players who are joining a Behest to not be in a party already...
Auto group the players who join into groups of 8
Each group will have their own mobs to attack
They could increase the behest time back to 1 hour if they do it this way.
Please implement this SE, seems so simple, yet very effective.
No one gets blocked out, people get to play and maybe even meet other people.
Another idea behind this might be the ability to sign up to the behests not only at the camps, but also at "Special Behest NPC's" where you can sign up to do any behest anywhere in the world from any one of these NPC's, and once you are done, it teleports you back to the NPC where you signed up.
Another spin-off that could happen is the ability to do behest every hour, but maybe not always on the hour. Where once 8 people sign up, they do the behest at that moment, not always waiting till the top of every hour. Then once its done, you must wait 1 hour to be able to join another behest. You would see the behests and how many are signed up and waiting to go. This way you can put your name on a list (sign up) and then go do something else. Once 8 players are signed up, it gives a warning that in 2-3 minutes you will be teleported to the behest you signed up at. Once done, teleported back to the sign up npc.