How to interrupt a loop with multiple players?
The described situation is perfectly legal. These loops are governed by rule 720, concerning the taking of shortcuts.
CR720.2a, b and c explain how shortcuts are to be taken:

720.2a At any point in the game, the player with priority may suggest a shortcut by describing a sequence of game choices, for all players, that may be legally taken based on the current game state and the predictable results of the sequence of choices. This sequence may be a non-repetitive series of choices, a loop that repeats a specified number of times, multiple loops, or nested loops, and may even cross multiple turns. It can’t include conditional actions, where the outcome of a game event determines the next action a player takes. The ending point of this sequence must be a place where a player has priority, though it need not be the player proposing the shortcut.
720.2b Each other player, in turn order starting after the player who suggested the shortcut, may either accept the proposed sequence, or shorten it by naming a place where they will make a game choice that’s different than what’s been proposed. (The player doesn’t need to specify at this time what the new choice will be.) This place becomes the new ending point of the proposed sequence.
720.2c Once the last player has either accepted or shortened the shortcut proposal, the shortcut is taken. The game advances to the last proposed ending point, with all game choices contained in the shortcut proposal having been taken. If the shortcut was shortened from the original proposal, the player who now has priority must make a different game choice than what was originally proposed for that player.

Notice the emphasized sentence: that player, in your situation, is Chris. Since Chris was the one who suggested the fewest number of iterations for Alice's loop, when he receives priority, he must take an action that is different from the action originally proposed by Alice for the shortcut.
As you can see, as long as it is a different action from the one originally proposed, it doesn't matter what action that is. Once that action (in your case, crewing the Aethersphere Harverster) has been taken, play continues normally in turn order. Alice can then propose a new shortcut to continue her loop if she wishes to do so, but that action is not required.