Morning,
I cant speak for the JOSM team however it is 100% implied. It makes sense logically and its stated on the wiki. This might have just been a oversight by them. https://wiki.openstreetmap.org/wiki/Tag:crossing%3Dunmarked
"crossing=unmarked implies crossing:markings=no. Adding crossing:markings=no is possible, but not necessary. Any crossing=unmarked with crossing:markings=* present and set to a value other than no could point to a mistake in the tagging."

I don't see how you could have a give way against traffic. Its a contradiction, for a backwards giveway you need to have backwards flowing traffic, on one way roads you do not, either the giveway is wrong or the oneway is wrong. The only exception i can think of would be some extreme case where oneway=-1 is needed but I would say direction=forward is implied on one way roads.

again i'm just nitpicking here, what you've added isnt wrong but just of clutter which you seem to be interested in reducing. 

 