This is very easy to pull off in game. Lethality sin requires you to hit your q, and that's just about it.

Great question. A well timed Ladder Insec will do the trick for you. If you don't have flash, do one of two things. A; wait for the tank to get close to the squishy (so you can q to them then perform a regular insec on the adc) or just wait out the q until it runs out.

Another good question. An insec is where you q on someone, q to them, wardhop behind them (ward behind them, w to the ward), then ult them. This is used so frequently because it gets someone (preferably a squishy target) to the clutches of your team.

You're actually right. A normal insec usually will get you killed because of how squishy you are, but if you get your lanes ahead it's normally worth it. If you for some reason find you're the only one on your team fed, do what is actually called an "In and outsec" or the "Safe Insec". This is quite complicated, so be prepared to practice this in practice tool. Q, Wardhop (ward toward them then w), kickflash (r then flash behind them to cancel the animation of the kick), then take your q back. This accomplishes the same purpose of an insec, gets you back to safety, but at the cost of your flash. Think about it this way: One flash to win your team the fight. Worth.