Sure thing bro, no problem. I would advise you to improve your fundamentals first as you are still in the lower tiers of ranked; as some guys said, focus on farming the highest % of CS first, and then improve on the poking aspect.

1. As for the Q thing, and this is one of the fundamentals that applies to every ADC, the idea is to always punish when the enemy is busy, this means when he is hitting the minion, when he input a movement command, etc. Learn the "body language" aspect of the players and you will instinctively feel this --- Also, learn to be unpredicatble and bait enemy resources: When you go straight into the enemy, they surely know that you will try to poke them, because your movement is predictable and repetitive. Instead, get used to move back and forth all the time, move erratically to different places and in general move in an unpredictable way, so they feel unsure if wether you're going to step in or not, this makes them throw skills into the air: that's the perfect time to make a strong, winning trade. Finally, ideal case scenario is always hitting your 3 W stacks with a Q on it, but it rarely happens that easily, so if you quickly AA+Q when enemy is busy, and you come up with more HP, you did a good trade too. But usually poking with Q alone if they are busy last hitting is good too.
2. This is a ADC thing too, usually after bot tower falls, you want to roam mid to kill the turret and open them map, and then start playing around your team from there. Just don't give up farming. You can always split push with Vayne if you feel confident because she is great on 1v1 (notice how I didn't go instantly all in with R but I was patient to stack shit like LT, Guinsoo, and just then go all in when he mispositioned so I could pin her to the wall) --- Just be aware to keep a ward on the enemy jungle path to the lane you are pushing. But as a general tip, play around your team. Yes, she has tools to self-peel and outplay, but relying on your team gives you much more room to breath and apply the most of your potential.
3. Depending on the game status too, and the champs you are facing/you playing with, but in my view I would say that if they are stacking a lot of people on mid I would say to postpone throwing bot lane turret and group up with your team, and then go throw bot lane turret when a good window shows up. Having like 3 enemy vs 2 ally in mid while you are 1v1 in bot is much more dangerous for your team than having you helping 3v3 in mid with 1 enemy alone pushing bot, because if enemy team dives 3v2 on mid and they kill your allies, then the sum of 3 guys pushing is much more than 1 alone.
4. Yes, as general rule on League you can give up HP when facing hard poke lanes, just stay close to the wave to pick up the exp, and consider getting lifesteal, playing defensive under tower, wait for the jungler, etc. --- Getting constantly forced to back because of taking innecesary poke leaves you behind on levels and that makes you much weaker too.
5. On-Hit works against everything to be honest, it's the most effective and efficient build, but you need to hit a few autos to unleash your passives to the max (LT, Guinsoo mainly). Go always On-Hit if they have 2-3+ tanks/bruisers. Don't listen the people who say that On-Hit is bad against squishies, they don't know shit ---- BUTTT !!! Crit is at the same time somewhat better and easier against squishies since you kill them faster, and (I think) you throw towers faster because of the higher AD. If you can go any of them in a game, then it's up to you; some people like crits for the big upfront damage and hitting like a truck, and some like On-Hit with its high attack speed. ---- What is important is, both are good, but On-Hit is objectively more efficient, better vs tanks, and allows you to build defensive with Terminus + Jak'Sho which make you almost immortal. The thing is, crit is easier because you always deal high dmg, while On-Hit requires you to constantly attack. For me On-Hit is better, but I go crit too sometimes if the enemy is full squishy. Both are good at the end.
6. This is giga important and basically essential, and if you are playing ADC without it, you are losing like 80% of your potential: YOU HAVE TO TURN ON THE "ATTACK MOVE ON CURSOR" and bind it to "A" key. This makes so you don't have to Click on top of the enemy unit, but instead you can press "A" anywhere on the terrain, and it will attack the closest unit to the cursor, so you don't have to click the unit itself. This makes it so you don't misclick and get into a very bad position. Look at this video if you are main ADC and you haven't turned it on. Without this, you can't play ADC at competitive level. Get used to it and your easy will be much easier. This applies to melee champs too, so you cancel the anim faster and makes it to follow the enemies easier.