What's the proper thing to do if one of your teammates engages when your whole team is out of position and has a huge chance to lose the fight?

I feel like it's smarter to just back off, 1 kill is better than an ace and an inhibitor. And your chance of winning a teamfight starting off 1 person down is so much less than a 5v5.

Depends. Is that one person Voyboy?

There is no answer to this question. Too many variables exists within any situation. My recommendation is to go to twitch.tv, watch the tournament, watch the pro streams. This will give you the knowledge to know when to go in and when to stay out.

No average game/ranked.
please don't troll.