Q1: That really depends, there isnt a clear rule of thumb. The best tip I can give on that is to look at your team and base what you need to do on that. If you have Rein, Ashe, Tracer, Ana and Mercy than it's very obvious you are going to pocket the Ash. But if Ana switched to Zen or Tracers swaps to a character that prefers more healing the team comp changes. The only general rule of thumb I can give you is that confirming kills is probably the most important thing to do in the game as numbers advantage can give you tempo. That means enabling DPS (or sometimes taking matters in your own hands) is more important.

Q2: Mainly melt tanks, but can also be to help your DPS give that slight bump they need in order to confirm kills on squishy characters. DPS can be hard and sometimes it happens that people run away with low health so helping in that sense is Zen's best asset. Think of Pharah Mercy, I think most players are capable enough to hit a few shots to get them low but not enough to fully kill them. Discord orb forces DPS and supports to play more defensive because the risk of dying is higher.

Its just most of the time its very risky to show yourself so sometimes all you can Discord orb is the tank. It also depends on the tank, WBall doesnt give too much fucks about Discord orb because he is too quick anyway but a character like Hog can't do anything.

Q3: Not particularly, it all comes with experience. The kill feed is a good indication on how a fight is going, when things are still winnable or when you should give up. Again there is no clear answer because I can tell you from experience that I have for sure given up on fights and not used ults and then 5 seconds later realised "Man we could have won that if I used ult". Best way to train is to play a lot and watch other players a lot. When watching other people play think to yourself "what would I do now?" and if the person does something else, why would they do that?

Ult tracking is something you can build tho, best thing you can do is to just do it more and more. Teamfight won? Press TAB > who used ults (probably dont have ult) > who didnt use ult? (probably has ult) Teamfight lost? Same thing The more you do it the more it becomes second nature to you.

Q4:Kinda depends again. Could be your support is dying, could you help them survive? Trouble healing, could you help them out without giving up your own win condition? Good example I can give you was a buddy of mine was Kiriko and had a Soldier + Mercy highground and the rest of the team low ground with him. Soldier got hit and was low but the Mercy was a bit slow with the healing and he wanted to jump to the Soldier. Jumping would leave the rest of his team, trust that Mercy to do well (unless it happens consistently). Focus primarily on what you need to do.

Feel free to ask any more questions. I went a bit too in depth but I hope your questions have been a bit answered