Scenario #1: This seems to be a very common problem (at my elo, at least) where people and teams do not know how to play from behind, so I'll toss out a few ideas here. Bear in mind I'm a support main so I look at it from this perspective.
- If you lost your lane/turret, then you help the jungle reclaim his camps in whatever fashion you can. You have effectively adopted a support role at that point and have less impact on the game that your direct opponent (this is highly generalized of course, and is not meant to be a permanent state). You ward, you help him contest, you follow him if he decides to invade, etc. Additionally, you can try to impact the one lane that has not yet lost and try to establish an advantage there.
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If you are in the one lane that has not lost it's tier 1 turret yet, you ward the gank paths the enemy will take to you and then you apply as much pressure on your lane as you can safely apply. The goal here is to maintain what little advantage your team has, but you have to preserve your own point of power; meaning, if the enemy team comes to take your tower too, and your team is not in a position to defend it, then you concede it as well without giving them a kill. Losing towers sucks, but dying and then losing towers sucks more. Also, if you are still up then you can either reset the lane or potentially contest neutral objectives.
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You ward where it is safe to do so, and then expand your ward lines from there. If you can only ward the bush just outside your base gate, then you ward the shit outta that little banana. Then, when you know then enemy has pulled back some, you group as best you can and push the ward line out. At this point, vision is the only thing keeping you safe, and if you don't have vision in your own jungle then it is no longer your jungle. A more complete answer to the question is this: priority goes to the gate bush, raptor camp bush, and the bush near the first jungle entrance as you enter Brambleback jungle from blue side bot lane. On the other side of the jungle, you ward wolves, the brush between wolves and Sentinel, and then the big intersection between Sentinel and Gromp.
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You only take fights you know you can win, and this actually plays off of both of the other answers. You nurture whatever advantage your team has and you protect it with vision. When that vision reveals that you have a potential pick, you take it... ONLY if you are quite certain of winning it. This is in between potential contests at neutral objectives and passive lane farming. The idea is to look at the strongest point of scaling power on your team (you have a capable Ryze mid/top? Or a Vayne?) and to funnel as much farm into them as possible.
There are two things this meta has proven to me, and I see both of them very regularly in my elo. It is very, very hard to play from behind with the power of snowballing from first turret blood; and people still don't know how NOT to overstay. The second point is where the comeback lies, but you cannot overlook the advantage 1200 gold brings to a team in spite of you getting free kills from their overstaying mistake. You have to punish every possible mistake, and you need awareness of how to contest or steal neutral objectives to remain relevant.