Well, I'm a pretty scrub 4k player, but I consider my LC jungle pretty ok.

I provide extensive answer for your first 2 questions, as I think it's really game dependent, but generally when you need a good single lockdown for heroes with good escape(like ember or AM -even though player with good reactions can remnant out/blink before you can duel) or against rats with summons(deffinetely vs brood), or when you simply feel like playing it...

You can jungle her when your team has good solo offlane and support that can handle early game without much farm(like CM) and your safe lane can withstand enemy offlane, however, you should help him out with wards later on during the game(usually i start buying wards after i get blink).

You deffinetely shouldnt jungle her when enemy has(or there is possibility they will pick) some roaming shit like bounty or pudge, or when they are going some dual offlane that rekts your safe lane, since they will try to find you and demolish you, rendering you USELESS for another 15 minutes at least.

As for skill build - I go 1-E, 2-W, 3-Q, 4-Q, 5-Q, 6-R, 7-Q, then it depends on the game, but rest is not as important, you take ulti at 11/16 and W and E as you feel like during the game, but generally you max W after Q. You may ask, why max Q if I am jungling? Well, using choke points in jungle and using your 1 point of W should be enough for you to get you to lvl 6 with at most 1 trip to base in between and you want to be online ASAP when you get 6.

As for item build(for jungle) : Starting items - Iron talon, tango. Start on the big camp, cut tree in before, to make choke point, use talon on big creep. Rinse and repeat for big/medium camps. Use tango when you need, just dont save it till you are low hp, since it regens slowly. Once you run out of tango or when you are simply low, dont hesitate to use W, preferably WHILE attacking camp, so you get use of that attack speed bonus, not in between camps. When you run out of mana and tangoes, go heal to base.

Try to farm boots asap, it helps you move between camps faster, also faster trip to base and back. After you just rush blink. You should get it around 7-8 minutes if you are uncontested and you should get your level 6 as well. When you get lvl 6 and see possiblity of ganking lane and winning duel even without blink, do it. Otherwise, just farm your fucking blink as fast as you can, since your team is playing 4v5 and waiting for you to come online.

After you get your blink, you need to win a fucking duel, just getting a kill is not enough, so buy also smoke and ask for help from your team, they should be happy to help you, since you are finally doing something. After boots and blink, finish your boots into PT. Then I usually go for blade mail, then AC and bkb - order depends on enemy line up. If you can afford it, go AC first, if they have too much lockdown/magic shit, go bkb first. After this, Its all game dependent, but items such as basher/abysall, mkb(vs evasion) are my usual choice.