MDH and MDV 0% is the same as long as the aspect ratio is also the same. Any other percentage will be different though, as well as converting between different aspect ratios.
The best way would be to use 103 HFOV for Apex as well (1.237600), then it will be exactly the same with both methods.
If you want to use a wider FOV, just try both methods. One of them will probably feel too fast or too slow.
The difference between 102.993 and 103 is most likely less than one count. Around a quarter of a pixel, it's basically impossible to detect this without extremely accurate measurements.
Viewspeed horizontal is about 124%, viewspeed vertical is about 133%.
The calculator actually does the math for you, if you select a scope and viewspeed, the placeholder for USA will show the approximate equivalent.
Technically yes, but since there's no separate ADS sensitivity setting it will affect your hipfire sensitivity. I'm not sure if different weapons have different ADS either, but if they do they would all need their own calculation.