I'm not a super high level player but I am pretty smart.
There are alot of variables that need to be considered before making the proper decision for some of these situations which means there's no end-all solution to that type of problem. So I really can't answer all of these perfectly because I would end up writing a books worth of information for each role and champion. But i'll answer what I can
1.Too many variables. (how fed are they? How fed are you? How fed is your team? How fed is their team? How far pushed up are they? How many towers are up? How much hp does the tower have if it is up? What role are you playing?)
-
If bot is 0/10 Chance are, if you go down there you'll probably die. However, if you have a hard engage support like thresh leo blitz you can probably gank if they land their cc if not just back off or wait for the enemy to engage before hopping in and if someone doesn't die instantly then you can probably get a kill/summoner spell out of it. If you do get a summoner out of it they'll probably ward the bush you came from if not both of them. As a jungler you need to be as crafty as possible with your ganks so don't be afraid to do a lane gank (Via the bot lane brush) and if they are super extended then you can probably just gank them even if it's warded just try and stay out of ward distance and watch for an engage on either side before you go in.
-
If you are feeding as long as you have cc you're still useful, and you should probably participate in fights. However with some champions (Yasuo, Zed, or pretty much any top laner.) you want to split push as much as possible and tell your team not to fight and just defend while you do so. You'll get back in the game with the cs from split pushing and you'll be still create pressure on the map. But if your champ can't split push then use your cc to peel for your carries and/or the most fed person on your team.
4.You will need more than one person mid, however you don't have to go mid and you should probably keep pushing bot/top even after you take their tower eventually because then you'll force someone up there.
- (for some reason a 1 appears here instead of a 5 even though there is a five typed in this space. O_o)If people are arguing then try calm them down because people don't play right when they are mad or salty, Don't ever tell them you will mute them because that will just make them more angry. If they refuse to listen and even throw insults your way don't be afraid to mute and report them.
6.Defend mid the best you possibly can, get a few more people mid but at keep at least one person both top/bot. If you do this, you can probably defend the tower (higher level players will dive 5v3/5v2) but in lower elos most tower dives(edit forgot the dives in tower dives :P) aren't executed properly because some can't land their cc. Your team will take more objectives in 2 lanes then they will trying to contest just one (considering you called 1 or 2 people to go mid) and it will eventually force them to go top and bot to keep the minion waves back or to keep your team from shoving down to their inhib.
7.Probably the one with the most variables, from how fed they are even down to the champion it's self and their specific power spikes and weaknesses. I really can't give you an answer for this :/