Totally agree with you, I was posting the base rules that I would apply to all roles and players.
Of course, you need to adapt to the champ that counters you but also the enemy team make-up, their CURRENT strengths and your own champ.
For example,
If I go amumur, who is a tank, but can be built with pretty strong AP, Leaves him not as squishy as other champs but squishy enough to make my point.
If I'm against a multi AP team, I would start building MR. Now, if 12 minutes in I see that the AP champs have basically been feeding, but the ADC has seriously gotten fed, I abandon my build choice and go full defense.
Now, if my style is like yi, then I'm a ks'er all game long because that makes me strong, but I'm also CAREFUL who I steal from. If I notice for example I have a cho-gath top, well Cho doesn't need many kills to get the "Ohh hell no" tank level, he needs a bit of farm and a decent amount of gold, but not like a graves, Lucian, ahri, Karthus, and so on. In his lane, I would steal the kills, knowing I'm not hurting my tank, but rather buffing my AD ability.
I would not however go bot and steal a kill from Kalista(for example), as that would hurt our team as a whole.
The dynamics of good decision making come from not only understanding your champ, but either communicating well with your teammates or completely understanding their champ and their style of play.
As a support, you're right, you need to peel in teamfights, like Janna has a potentially dangerous as fuck ult. She can split a team, fully disengage, or simply keep her ADC alive long enough to ace the enemy team. Almost all supports have some method of doing this, but on the other hand, they cannot be held to the standard of making huge plays at every teamfight, and if you depend on others to make the engages for you and to give you the kills and get you fed, then you are being carried. Any ADC who sits back and lets his team do all the work but manages to get he last hit off is not carry'ing, they are being carried.
I do disagree with you on one point. Any position can be a full carry, You can't 1v5, lets face it, but you can make smart decisions.
If , for example, you see a braum jump right into the middle of a 5 man grouping, and pops his ult before the team even gets in position, then you know, that is a player who makes bad decisions, don't go in with him and let him know, you will not be backing his plays, unless he actually makes a good one.
You're looking at carry'ing like it's the person with the most kills or assists. I play lulu support a lot, and thresh and Janna, and I can tell you that my stats most definitely do not reflect my involvement. I'm a bronzy, so not going to say I'm good, but I will say that when My ADC gets ignited and is on the receiving end of being hit by the other ADC and I throw my lantern out to provide the shield and that saves the life of my ADC, then we turn it around and I get no assist, trust me, I made that kill possible as much as if I was to hook the guy.
I'm no carry, but the play maker, the person making it possible to win is the carry, regardless of KDA. An amumu who ults the whole team and dies because of it, but we win the teamfight following was the carry on that fight. Don't let KDA full you, it's not the full story.