The method you use for group is fair since you're encourage your group to donate to contribute to the group fund and you are rewarding those who participate in GNW but has some drawbacks. If you happen to have players below level 85 and if they were to donate max every day in comparison to someone who max donate at level 85+, the players below 85 won't get a pack since level 85+ can max contribute more. A solution would be adding a cap on a player to give a pack someone that is below in the queue of contribution.

The mail method is a bit lazy on the group leader part since he doesn't want to put in the effort to reward those who make a difference in the group. He might end just packing it to his favourites in his group.

The fairness method is fair since you're making sure everyone has a chance to get a pack but has a downside. The inexperienced players might not use their seal scrolls appropriately such as spending on treasures that don't fit with the meta or don't spend it during a rebate.

The top priority method insures that the group be #1 by making sure the top players get the seal scrolls to get ninjas like roshi, han, etc. From what I read, some pass on the packs once the top players are done with their lineups until a new treasure comes and they get back on top of the queue. The lower players might not participate if they aren't appreciate for their efforts. This seems this method works if the majority in the group is p2w/top-tier f2p to the point that the lower players contribution don't mean anything.

Each method has it's pros and cons. What matters is which method works best with the type of group you are. If I were to say which is fair out of the methods you have said, the one you currently are using is.