So basically OP you are complaining that if someone could leave, the replacement could be the same job (or type as you i.e. melee) and that you then have to compete for potential loot? I mean this happens in a ton of dungeons not just 24man, and they have just as much right to roll on that loot considering they did accept to be the replacement.

As for loot distribution, you have two options, either enter with a pre-formed party that way you can allocate stuff however you want, or just ask. Seriously ask your group if they are willing to pass on something you might want, granted it doesn't always work but you have absolutely nothing to lose by trying as most players will be happy to cooperate.