MDH and MDV is essentially the same, but you have to convert between them using the aspect ratio of the monitor. So for instance MDV 100% on 16:9 is the same as MDH 16/9=177.7778%.
The benefit of using MDV is that you usually want to keep your vertical sensitivity the same regardless of aspect ratio. Imagine a square box that covers the middle of your monitor, you want the sensitivity within this box to be the same. If your aspect ratio is wider, then there's just more to view, but the sensitivity within this box stays the same.
If you use horizontal, then going from 16:9 to 21:9 will drastically change your sensitivity, instead of just keeping the same sensitivity within the 16:9 portion of the 21:9 monitor.
The reasons for going above 100% is usually just to match your preference. If 100% is too slow, there's no reason you can't try 200%. It's correct that it's matching a point outside of your monitor, but it's still a valid and consistent conversion.