There is a confounding factor: Do the rules for
Moving around Other Creatures apply here?
If yes, then to manage that, you would need to use the
Grappling rules, namely the effects of the
Grappled condition, which means the speed would be capped at 30, not 60.
So, that would mean 30-feet per turn moving the Rogue (unless the Rogue is smaller), for 2 turns, then back in 1 (3 total); followed by the 5-feet per turn for the Cleric for 12 turns, then back in 1 (16 total).
As for moving the Fighter, you are correct that the Wizard could not move them RAW, due to the rules for
Carrying Capacity capping Drag at 30x Strength. For that, the Fighter would need to hand 6+ lbs of stuff to the Cleric, which would allow the 12 turns to move the Fighter (28 total), well within the duration of the Fly spell.
As for casting Fly on the Fighter: Unless they have a lower Strength than the Wizard, it would probably be easier just to target the Fighter in the first place, and let them ferry everyone across.