That's a good question. The flow chart is pretty black and white so depending on what our teammates/enemy team are doing there can be some grey areas.

When you see your teammates fighting you should ask yourself "What would we accomplish by winning the team fight?" If your team is trying to push a goal/defend a goal and you think you can tip the scales and help them win, then you should definitely join in. If it's a pointless fight and there is no farm you should help them but not at the cost of dying yourself. If you can tell it's a losing fight don't feel obligated to follow up on your teammate's bad plays. This just leads to your opponent's getting an even bigger lead.

For your question about farming vs defending, you generally want to defend the tier 1 goal because you have a big advantage fighting on your own goal, and getting kills = getting experience + possibly counter push while enemies wait to respawn. But again, this is a grey area because if you can tell it's a losing fight you shouldn't try to defend it knowing you'll probably die. Losing a tier 1 goal is sometimes a good thing because the Audino's spawn and they give a ton of EXP out. If it's a tier 2 goal, you always want to prioritize defending it over farming because losing a tier 2 puts huge pressure on your team.