I understand this will be added to her earned income and if her earned income is over the threshold she will pay taxes on the difference. correct
Is there ever a reason where her taxable scholarship income should be added to our 1040 instead (husband and wife filing jointly)? never
Does the ACA tax credit require her income to be added to ours? if she is required to file a return then yes
Again, my understanding is that if she is under the threshold for filing then her income does not need to be calculated into our MAGI for the ACA credit. again correct ... follow the screen instructions. If she is on your 1095-A form then it must also be listed on her return and the credit must be allocated in any way you choose so don't file either return until you decide who will claim what %
