Yes, a shinobi must demonstrate the ability to use jutsu before we include it in our debate. Adittionally, being born with a nature type doesnt mean you will have the skill to actually USE the technique, I see your arguement here. But, I don't think you understand the Nature of KKG.
The link above describes the usage of Kekkei Genkai. But in order to use a KKG jutsu specifically, you already have to have the ability to manipulate each jutsu seperately
. You do this in each hand (one nature type per hand) and combine them both to create a new Nature Type. But you need to understand that there is no such thing as an "Ice Nature", "Wood Nature", "Boil Nature", etc. Shinobi using these jutsu are actually using 2 of the 5 nature types simutameously
--which means Haku's ability to manipulate WATER and WIND is so advanced that he can use each technique with only ONE HAND
. Kakashi says in the link above says that KKG shinobi can control two different elements AND
the ability to combine them both to create something totally new. If you need MORE evidence:
Kakashi says (above link) that Haku has the ability to "...control(ed) two elements
, WIND and WATER
, to create Ice." If this is STILL not enough evidence to suggest Haku can manipulate more than just "Ice" Release...
...he actually DID use a B-Ranked Water Release jutsu (above link) called "Thousand Flying Water Needles of Death"
in Manga #24, First Databook, page 205. That's a Water Release jutsu performed WITH ONE HAND, just as I described previously
. BTW, he was born into a village known for using Water Nature, and was trained directly by one of the most proficient Water Release shinobi in the Manga