1.) Is it more important to have a full build rather than prevent ganks?
-I don't believe a full build would be enough to counter a 2v1 gank
I'm not sure I understand this question. When I mentioned that you had no completed items, it was a combination of two pieces of advice: A.) You should farm so that you have enough money to buy items, and B.) You should generally finish one item before starting the next one, because completed items are that much more powerful. Simply concentrating on a single item is not enough to fend off a gank, but farming up is. It definitely is. If I were to go top in a low Bronze game and concentrate entirely on getting 4x my opponent's farm without even bothering to kill him, I'd be able to 2v1 when the jungler came. That's the power of having a large item advantage.
2.) How can I assist my team as a jungler, whilst still maintaining a good farm when all lanes are losing? (btw, i ended with the 3rd highest CS of my teammates with 109)(higher than our mid & ADC)
-If all 3 lanes are losing, how can I help them (without losing farm).(Teleport helps me a lot)
This is a judgment call. It's important to first ask, why are all lanes losing? Is it because of bad matchups, bad luck, or bad players? If possible, go help the lane that lost because of bad luck, because they'll be able to benefit the most from your help. You also want to question whether there's something you did wrong that caused your lanes to start losing, so that you can avoid it next time. If someone is in a lane where the opponent has a large advantage over them early, consider making that a priority right after your buffs. If someone seems to be overextending and you suspect they will be ganked, head there to countergank.
Maintaining farm while helping lanes is a matter of moving around the map intelligently. Never dilly-dally. It helps to have a sense of urgency. Put this song on loop in the background. Go to a camp, clear it, make a beeline for the next camp, clear it, then run to the lane that's most appropriate. Don't stand around in a bush hoping that a gank opportunity opens up, and don't hang out in lane killing minions if you could go for another jungle camp instead.
3.) Is farming more important than losing a lane?
-Should I keep in the jungle while their Jarvan is camping a lane?
This depends heavily on who the junglers are. Jarvan has strong early ganks. It makes sense that he will gank early and often to get his lanes rolling. Yi (whom you play a lot) is a strong farmer. While your laners may protest, it probably makes sense to keep farming rather than try to gank for them. Yi has no CC at all, just a little slow if you have red buff. Unless an enemy is very overextended without wards, your gank will fail. On the other hand, if you completely ignore lanes, there's no way of hitting your late game power spike because the game will just end. So in some sense, it is a judgment call. It is also a case where you'll want to pick an appropriate champion. If they have a strong early ganker in the jungle and they also have strong early game laners, a farming jungler who doesn't gank well will be a huge risk.
I actually spectated one of your games tonight, the one where the enemy had Ahri mid and Hecarim jungle. I noticed that you spend a ton of time in the early/mid game killing minions in lanes, and very little time killing camps. I was especially surprised to see you totally ignore your buff camps. There was even one point where you put wards in the bushes behind and inside your blue buff, without noticing that the blue had been counterjungled a long time ago. Your wards were defending an objective that didn't exist, and won't exist for several more minutes because you didn't stop to clear the tiny lizards. If you're going to jungle Yi, you definitely must buy a Wriggle's and focus heavily on farming, always taking your buff camps right on time. You would need a very good reason for deviating from this strategy on that champion.