Wasn't it confirmed he is an emitter? That is why he could learn Leorio's ability as they are both emitters. When Nen was introduced it was explained that a user can learn abilities from their same category and the adjacent ones to a lesser extent, but not from the opposite side of the spectrum.
This principle was never shown in the series until that point. That is is why people think Gin has the power to copy abilities, but it's just the fact that emitters can learn from each other.
His Nen type hasn't been confirmed.
He was able to replicate Leorio's technique on pure Nen prowess alone, just raw skill in the art of Nen. Because he's able to replicate techniques like this and because he's a top 5 Nen user, I think its fair to say that he's close if not already maxed out his adjacent categories to their potential mastery. Because of this, getting hit by Leorio's attack gave him enough data to replicate the attack. Even if Ging was a Conjurer, which has the lowest mastery potential mastery for Emission, he'd be producing Emission techniques at 40% efficiency but because he's such a prodigy his 40% is probably more proficient than Leorio despite him having potential for 100% mastery - its important to remember he's been mostly focusing on medical school. Wouldn't be surprised if Leorio has reached only around 20% mastery in his category.
Anyone can learn techniques from any category, it doesn't matter how far away on the Spectrum other categories are. It just means the mastery potential is lesser and its more difficult. The exception is of course Specialisation. Don't quote me on this in particular but I'm pretty sure it was stated that only Manipulators and Conjurers have a small 1% chance to also become Specialists later in life, because they're the closest to Specialisation on the spectrum. Enhancers, Emitters and Transmuters can't become Specialists.
Yeah that's how i read it from the beginning, he might not be the strongest raw power wise (or maybe he is who knows), but his sheer knowledge and talent in Nen manipulation makes him a monster.
I don't know, where do he falls in Hisoka's Personality Chart? He seems to perfectly into the Trasmuters category.
I'm pretty sure Hisoka ended up mentioning that his personality test can be unreliable.