Asking for help in improving my game

I have a couple of questions that I need answers for in order to improve. You don't have to answer all of these questions, I just need some advice from the community.I also try to answer my own questions that might need some contradicting if I'm wrong.

What do you do when there is a fed champion? If there is a fed champion that was fed early game from your teammates,what do you do? Instead of surrendering. I try to be more cautious when team fighting them. And maybe, if I'm playing assassin like , I try to kill them in order to balance out the fight. I think stalling could also benefit in decreasing the fed's snowball.

Follow up question, What do I build to counter Lethality? More Health? Being one shotted by a fed with full build Lethality is just scary.

When should I roam? I'm always afraid to lose CS, even worse, not being able to kill the target and losing precise time.

what's the fastest jungle path and what should be the level difference compared to teammates and enemy team? I sometimes feel slow and inconsistent when jungling, I always check my scoreboard to see that I'm two levels behind.

Thank you.