It's been done before on pistol mags, not an issue. I think they just went with the status quo.
I don't see how it would. However the fill valve on the bottom does make it a bit more comfortable/easier to hold when filling.
Yeah that's an unfortunate decision, fill valve at the back of the mag below the output valve is the best place. Quite surprising since GHK usually does that (AK, M4, AUG, 553/551) rather than putting them at the bottom.
To this day many Japanese companies have a "holier than thou" approach to foreign markets, that may have been fine 15 years ago when their products were head and shoulders above the rest ... not so much nowadays.