Since H2 only has 3.5 and 6.3 inputs, the answer to the first question is obvious...
About Wm1a and H2, I have both, plan to keep both and the answer to you question isn’t so obvious.
In SQ department, IMHO, H2 is (much) better than Wm1a. More detail, more “organic” sound, much better amplifier, etc.
But, with certain types of IEM’s like mine CA Andromeda, I suppose because OI of H2 being so low, I prefer use Wm1a instead of H2. With H2 although the great detail, separation, organic sound, etc, they become more bassy so I prefer use Wm1a (balanced) with it’s more “dry” sound although very musical.
For headphones like mine Mr Speakers AFO, the H2 is by far better. It’s really a joy the combination H2+AFO.
I have the Violectric V100 which i’m going to sell because since I have H2 I don’t use it.
With AFO, the SQ direct from H2 is way better than H2+V100.
I think for the vast majority of the available headphones, there is no need of a dedicated amplifier when using H2. Adding an amplifier will only contribute to SQ degradation. IMHO...