Background: I'm running an OSR-style Open Table game, which means that I don't have a traditional established party of 4-6. Instead, I have a group that technically contains 18 players- 9 of which have not yet attended a session but expressed interest, 2 of which have never missed a session, 2 of which have only attended once, and the other 5 are semi-regular.
This has a lot of advantages, but creates the question of how to equitably divide loot. I could divide everything completely evenly amongst all past and future party members, but if three people kill a dragon and then get 1/9th of its treasure each because we're setting gold aside for Steve in case he ever shows up again... that doesn't seem quite right.
I could divide treasure up amongst only the people at the table, but that feels unfair to things that happen across sessions- If Jackie decodes the map to the dragon's lair, helps outsmart the spiders of mirkwood, and finds the Key Golem to unlock the Dragongate, she's clearly contributed to the overall operation, even if she isn't there for the session where the dragon is slain.
On the other hand, a system where I have to maintain a running formalized list of every ongoing quest and everyone that's participated in those quests seems like a lot of extra bookkeeping.
I think if your group is that amorphous you would have to design your adventures/dungeoncrawls such that they can reach distinct conclusions on a per-real-life-session basis.
I know it's something we as DMs/judges struggle with perenially: how do you explain away somebody who made it to last session but didn't make it to this session? There's never going to be an answer that satisfies every need. In the last (not ACKs) game I ran, I had a player that missed so many sessions that I declared the orb of transmutation they found sucked him into a pocket dimension. When he actually made it to the session, the orb projected a corporeal version of him from inside the pocket dimension, and otherwise was dormant. there was no rhyme or reason to when it projected because it was impossible to build a story reason for how often they showed up.
The truth is, all your proposed solutions have exactly the drawbacks you've described: evenly dividing regardless of attendance cheapens the success of those who showed up; only giving loot on attendance screws over people who contributed earlier in the adventure; keeping track of ongoing questlines is a lot of extra bookkeeping.
Personally, I wouldn't accept solution 1 at all. To me that's a style of play that I don't come to ACKs for. There is a big risk of death that comes with ACKs, and being removed from that danger shouldn't really result in a level increase.
The second solution can be alleviatd by not making multi-part adventures where the payout is all lumped at the very end. If the dragon has multiple weaknesses that require a good deal of legwork to exploit, the dragon should be paying lackeys to keep those secrets safe such that knocking them over comes with its own bump in treasure. It's ok if jackie doesn't get to actually kill the dragon if the dragon cultists with the map have loot, the mirkwood spiders have loot, and the key golem has loot.
The third solution can be managed by trying to push some of the book-keeping onto the players. If jackie found and decoded the map, now she's a stakeholder in this undertaking and won't give up a shot at it. If you have players of the right temperament, you could say that their mistrustful characters split the map into equal pieces so that they can only embark on that quest when they're all there (with possibly some hangars on). If you frontload cues and maybe make a few props, you can probably allow the players to remember who was collaborating on which major campaigns.
I’ve been kicking around awarding half or quarter XP and/or treasure to players who don’t show. Not out of what the party that showed earned as shares, but as “well presumably you were doing something useful (but less risky) in your free time” to keep irregular players from falling too far behind in level.
Personally, I always leave loot distribution up to the players. It's much more fun that way.
At one point, the party's only dwarf declared that the magical sword they'd found belonged in a (swarven) museum, and insisted on delivering it to one. When that character and player later left the campaign, the remaining players began to ponder the feasability of a museum heist...
I had that problem. What we ended up doing was keeping a spreadsheet of who attended what session and then when they went back to civilization the total xp was divided up among a bunch more shares but people only got shares for how many sessions they played. It was extra book keeping and eventually we dropped it, but at the same time, a lot of the players who missed a lot of sessions stopped playing for real life reasons so it kinda self-corrected.
Say you have 4 players and 2 henches each. The dungeon takes 5 sessions to complete. Players A and B attended all 5 sessions, while C made 3 and D made 2. That ended up resulting in 15 full shares and 30 half shares or 30 shares. A and B get 5 each, C gets 3 and D gets 2 with their hirelings getting the same amount of half shares.
Like many ACKS devotees, I like spreadsheets. With a spreadsheet to automate complicated distribution of combat XP and treasure, you can also experiment with very proportional schemes.
For example, when my current campaign had more varied attendance, I tracked combat XP by session. For loot, I tried proportional and even. Ultimately, the attendance largely stabilized, and the players were willing to go with even distribution in order to ensure that the character of the least regular player didn’t fall behind (they wanted the Elf’s extra spells).