i checked the mobs when we just did a synth one. They conned ITG. And i can confirm that maze holder/party leader has no affect on level of mobs...b/c we used a level 1 char and they hit pretty hard to lvl75. we thought they were going to be easy and werent prepared/buffed and a couple of us almost died as we tried to solo before we knew wtf was going on.
we still ended up winning both. got double drop on second one. 35 marbles each. it appears to ask for synths based on the maze starter's highest craft...first was a 100 bone, 2nd 100 cooking (me)...first one asked for maze monger earring with bone synth stuff, second maze monger soup with (obviously) cooking stuff. no fails. i'm assuming when you choose to synth as a group it takes into account everyone's crafting levels (and possibly gives skillups to everyone). i'm guessing if u go in with someone near capped that this maze is easier since i'd guess failure rate would be lower.
as for drop rate, i'd say it's higher than 20% regardless...although i know some folks that have done like 6 in a row with no drops. i've seen about as many double drops as i have singles so i dont know if that means w/e the rate is...you have about 50% more chance to get another...maybe not quite that high, but i've done about 25-30 runs now. i've seen at least 4 double drops out of that