The problem with this move is more that it makes unworkable assumptions about how it's used than that it inherently "crosses the line".
"Crossing the line", remember, is when you ask a player to take on the GM's responsibility of authoring the world. In general this becomes more of a problem as the move becomes more universally applicable, and it sounds like you're definitely writing a move that you intend to be universally applicable.
Viewed solely from the perspective of the PC in the moment, it sounds like on a 7-9 you're offering them a choice between:
- being on-target but dealing a glancing blow, or
- being off-target but dealing a solid blow
This is a reasonable set of options for a PC to pick between, at least if this is a move any PC can be expected to do. Remember that address the characters, not the players also means to say things that the characters would know; the soundness of your aim and the target you will hit are things a character would know, in the moment.
The reason this move might not seem solid to you is because of how you're framing the difference between these choices, and the assumptions you make about the circumstances of resolution.
What You Assume
A 7-9 is not a 6-, so its effects should be at least partially positive for the PC, but not as good as a 10+. As written your move is saying the following things:
- The most effective thing you can do in this moment is always to land a direct Hit on your primary target.
- It is always desirable to attract the attention of your primary target, but not as effective as landing a direct Hit; attracting attention is a lesser alternative.
- It is always desirable to land a direct Hit on something near your primary target, but not as effective as landing a direct Hit on your primary target; a direct Hit somewhere else is a lesser alternative.
When you wrote the move in the first place, you were probably thinking of some specific fictional circumstance where somebody's lobbing an improvised projectile and all these things hold. You have doubts about the move, and I'm pretty sure they're doubts that your assumptions won't always hold true.
What To Do Instead
If this move was spawned by a specific fictional circumstance that you expect the PCs to encounter soon, it's not wrong to write a custom move just for PCs interacting with that specific fictional circumstance. When you lob an amphora of blessed water at the necromancer's hulking bonewalker, or whatever. The less-abstracted side effects of a 7-9 will make more sense to the PCs in the moment.
If you want to make this move more generally applicable, then you have to generalize your 7-9 choices to things that will happen more often. I think the idea of "on-target but glancing vs. off-target but solid" has some merit to it, but what does "off-target" mean in a general circumstance? Maybe you could try this:
When you hurl an object at a foe from a distance, note its Hit and Splash effects; they may be predefined or the GM may tell you in the moment. Then roll +DEX.
- On a 10+, you Hit your intended target. You may choose to Splash a second target in Reach range of your target; if you do, the GM will Splash a third target in the same range.
- On a 7-9, you Splash your intended target. You may choose to Splash a second target in Reach range of your target; if you do, the GM will Hit a third target in the same range.
You and the GM can choose environmental targets to affect, not just PCs or monsters.
This move sort of follows the "take a risk to do more damage" pattern of Hack and Slash; note that the GM isn't limited to choosing a target that will benefit the PC. There's also some space here for mechanics that play with that choice, like volatile thrown items where you must choose to hit an additional target.