I don't know how much of the conflict here is different value systems and how much of this is different interpretations of the psi system. It could easily be either.
Let's try to clear up any interpretation differences a little. Suppose Amu tried to use Open Heart against Kana here, in whatever way makes the most sense to you. And suppose she succeeded. What effect would that have on Kana's mind in the short and long term, and how would it produce that effect?
My immediate thought is "She wouldn't succeed, because it's a type mismatch". Open Heart does one specific thing, which isn't applicable to Kana; and the
way it works means it'll fail cleanly instead of doing something Amu wouldn't want.
(She also needs the Lock to pull off an Open Heart proper, but Amu could do a decent approximation; it won't change the outcome.)
Let's say she succeeded. This would imply that Kana fits the criteria the Lock is looking for, which at the present moment isn't
completely implausible. Much less so than it would normally be, at least. In the process of succeeding she would temporarily strengthen the bond between all of Kana's constituent parts, which would in this scenario (by definition; she succeeded) avert the shadow fight by way of keeping both parts of her too sane to want to do that.
It has few long-term effects per se. The initial effect of the heart beam wears off in a matter of minutes, and 90%+ of the lingering effects are due to the impact of the memories of that state. The remaining 10% is due to the enforced state of lucidity, not the beam as such. Seeing your idealised future as a chara, capable of talking to you, tends to be hugely impactful on the children she does it to; but it's about as natural an influence as that can get.
(There is no chara here. Like I said, this is a type mismatch. If it worked at all it'd work by enforcing a state of lucidity, postponing the shadow fight, but possibly only for those few minutes until it wears off unless Kana manages to come to terms with herself in that time.)
The underlying mechanism for all of this is, of course, mind control. Reaching into the targeted child's mind to hold it together, reduce stresses, potentially add detail to an incomplete chara, what have you; it's a very
specific sort of mind control that's probably the best the Lock's designer could think of, but you're still reaching into someone else's mind and turning knobs to do it. Even if 95+% of the time the Open Heart 'attack' was being used to heal psychotic breaks, even if it was the best option available and any other choice would be dramatically worse, it's
still mind control.
...
And it was impossible for Amu to do it as many times as she did, without picking up a trick or two.