im an adc main in Gold1 season 4 and Gold4 last season cuz i didnt play much, but lets see if i can offer any help.
When the enemy team is almost always in your lane, theres not a whole lot you can do about it except try to focus on what ever farm you can easily get without getting out of position.
Against strong poke lanes i like to stay a little further back than normal in lane, and i only come up close to the minion wave to get a cs...or stay behind minions if its someone like EZ who cant hit thru minions. Or...if you know your jungler is very near and will be there if something goes off...you can keep a little more aggressive stance in lane...either for an easier engage on the enemy or to pick up more cs. Positioning is one of the most extremely important things when playing adc...not just in teamfights...in lane too. Your stance in lane will dictate what you can do.
You should ALWAYS prioritize your own farm over denying the enemy of theirs. Really good adc's can do both at the same time very effectively. But i wouldnt focus too much on trying to deny farm until you can farm consitantly good numbers yourself. And once you trust your ability to farm in all circumstances, then add in the zoning strategy to it. Zoning is an effective tool, but you have to be able to get your own farm at the same time. It doesnt do a whole lot of good if you keep the enemy adc to 50cs at 10min, but you yourself only have 58cs at 10 min.
Your last question (#4) is very reliant upon whats happening in the game. If youre getting shoved under your turret all laning phase and cant farm b/c of constant harrass, then id say let them take it so you can farm much much easier. They will most often leave the lane once they take your turret, and only come back once the wave has reached their turret. Giving up tower in this situation will allow you to basically free farm. If the lane is going the opposite way where YOU are winning...i like to hold off taking the tower for as long as possible. If youre winning then most often it means your way ahead in cs....so why take the tower and give up the advantage you have over them? (The easy farming).
There will always be times tho where you may want to do the opposite in these situations. Sometimes taking the turret very early, with the lead in lane, allows you to roam and get other towers in other lanes. Especially in this meta right now....having the turret advantage almost always turns into winning the game.
Try learning to read the game situation and ask yourself....if i take the tower now, what can i do with that? Or if i dont take it yet...how does that benefit me?? It can go either way. Same with giving up or holding onto your own tower.
I hope this helps in some way for you. I was bored at work and had plenty of time to write a detailed response lol i coulda kept going but this is getting long enough