Ideally your ADS should be based on the hipfire through a conversion, this way you get a consistent aim regardless of the zoom of your scope.
In this case yes, but you shouldn't rely on being able to match scope for scope since most games won't allow you to do this.
That should indeed feel about the same, you can always try to click the underlined sensitivity value to test this sensitivity in 3D Aim Trainer, it should be exactly the same as in the game if set up correctly.
That's not correct. The issue with BF3 is that it does not apply the same factor to FOV and sensitivity, so everything changes when you change FOV. And it's not the same across all aims either, so nothing really lines up.
Scale is great to match how games work. Say you in Apex set all the scopes to 0.7. You can replicate this in the calculator by setting it to MDV 0% (the default scaling of Apex) and scale 70% (which is essentially what 0.7 is): https://www.mouse-sensitivity.com/?share=6f2dcab9f58fe297ce9dc864922bf64d
This way you can be sure that whatever you convert to will scale exactly the same. You can experiment with it to adjust how fast or slow scopes are based on the scaling you use. The difference between this and just adjusting the Monitor Distance percentage is that this affects all aims in a linear fashion, while monitor distance will affect scopes more when the FOV difference is larger.
It doesn't technically transfer the sensitivity values as the calculations are done here on this site, but the FOV and 360 distance of the aim you click and your hipfire are transferred and used in the aim trainer.