I prefer tankier, but WW is a nice mix. Can take some hits, has CC, and has mobility (and sustain). I personally like skarner, he has good movement speed and can be tankier. He isn't going to net you a ton of kills or carry, but performs solidly. I don't carry as much with him, but I feel like he always gives me a consistent performance. He can manage a game and provide shutdown against a snowballing enemy. When you have a kat pop into the middle of the team fight and you grab her out of her ult with your R and then she dies, it feels great. Right now as long as you stick to higher mobility champs you can do fine. Eve is not quite doing as well but still does alright. Kayn I am half and half on. I have not tried Nunu since the changes, but he seems to do well. Shyv and Jax also clear fast enough to keep going. Jax is nice because he has the dodge and CC combo. I feel like he does well with the adc meta since he gets some dodge to burst them down before they can kill him, where other melees are getting slammed right now.
Some of your other questions have so many variables I am not sure on what to answer.
As for helping winning or losing lanes, it matters what champs you are looking at.
I will throw out my two cents on when to help a losing or winning lane.
If they are snowballers and they are already winning lane, get them fed, just make sure you dont get killed and kill their progress. With so many games ending early, getting those early snowballers running is a big deal. If you have a snowballer at top, then get them fed. Especially if you look at your competition and see someone on the other team is on a losing streak, put pressure on them cause they tilt fast. Warwick has an easy CC with ult and and E. If you can dive in and lock them down for your ADC to get fat go for it. If they are winning, but the champs are not snowballers and they have it under control, help in other places unless it won't take time out. Like if you have a tank at top, that does well even if they get behind a little, it is okay to let them be for a while. With Warwick you can move around a map so fast, that you can keep pressure on multiple lanes and keep them from trying to entrap your lanes.
If they are losing so badly that you would likely die if you try to gank, get mid or top to help. Don't just blow off a lane because they are feeding, especially if
it is bot because then your game will be over. You just need extra help and smart ganks to slow them down and give your ADC a chance. Warwick is great engage, but he is not one that can do it all by himself. If your team can't provide support and help, you may just end up speeding the feed. If they are losing, but the champ that is vs that lane on the other team doesn't provide much for their team later, do not stress about it, but make a stab if you have the chance. With a fast champ like WW, I am okay if it just makes them burn their summoner skill, because I know I won't lose much farm speed. With slower junglers you have to weigh the costs of how behind you can get if you don't secure a kill. If you spend a lot of time ganking with a slower jungler, but don't get kills you can get behind really fast. It may be bad of me, but if the champs involved with a lane are not going to have a big impact whether they win or lose lane, I don't worry about it too much.
I guess in the end, you just have to judge which lane is most likely to lose you the game. Most times it is bot, that is why so many champs camp it, but there are some champs at top and mid that should not be left alone, especially if you are against a JG that can carry a game. If you spend all your time bot, but the other JG is feasting on mid and top, that can become a problem real fast.
Overall, one of the areas I would say a lot of JG fail in is warding. Proper warding can answer a lot of your questions. And I don't just mean your jg, but the enemy's as well. If you can know where the jungler is all game, it gives you the ability to counter jg, to prevent ganks, and to keep yourself safe. If I know the JG I am against can out duel me, I make sure I am with my team when I counter, or always attack the opposite side of the map. Also, if I am keeping eyes on the enemy jg, when I go to gank top because I know he is farming near bot, I let my bot know and remind them to freeze or be safe until I can get back in. If I know the other JG has a weak pre-6 I ward their blue or red and will wait for them to get beat up by it and take the easy kill and the obj.
Sorry for the wall of text. I feel like JG is one of the most complex roles, just because you are managing the whole map. So there really aren't very simple answers for it :/
