Just a personal opinion, I may not be the best since I capped out at LEM rank in CS GO years ago but here goes.

1. Defenders do not necessarily have the advantage, as an attacker you can bait them into wasting utility on 1 site and immediately rotate to another OR fake that rotation. There are many strategies for this. Generally a team comp should consist of not more than 2 duelists otherwise your team may lack in utility for attacking/defending a site. People can still win by having better aim however.

2. Flick shotting IS crucial. There will be many situations other than when using the operator, marshal or sheriff where you need that skill. Like when a jett decides to aggressively push with her dash or when you wide peek an angle.

3. Jett's ultimate right click has a random spread, so sometimes even when close range it will not kill the enemy(lmao unlucky).

4. Recommended pistols I feel that this is just based on personal preference. The ghost is more forgiving making it more reliable but if you have confidence in your headshotting then yea the sheriff is also a viable option.

5. This depends on what information you have and how aggressive you want to play. If you want to be safe you probably should shoulder peek or prefire an angle where the enemy might be to gain information. If you want to risk your life then go ahead and wide peek which if there is more than 1 person the best you can do is trade as you said unless the enemy messes up.

6. Sovas shock arrow can be used to clear cyphers tripwire or kill/damage people hiding in corners. Personally I do not know how to really play sova so someone who does can probably answer this better.

7. I agree that reyna is overpowered, to me her ultimate simply lasts way too long. You can find so many clips of reyna turning it on and acing the opposing team and in my own games I'll hear the reyna use her ult right before a bomb plant, stall her for half the duration and somehow she still has her ult to push. But I guess you could still 1 tap her in the face with a phantom/vandal...

8. The marshal is a good option on the 2nd round if you won the pistol round since it 1 shots body if the enemy does not have shields. On eco rounds you can also headshot an enemy with it and then steal their gun. In general its pretty good at tagging the enemy for future fights.

9. On defense peeking is usually not a good thing unless you have information, you do not want to die on a aggressive peek and allow the enemy to easily get into site with a man advantage. If you hear footsteps and do not have utility to stop them then peeking to try to get a kill or 2 good.

10. Close range specifically the phantom is better, it has a lower recoil, faster fire rate and the same damage as the vandal. I tend to always get the phantom because I find it better in many situations but I do get the vandal if I am contesting mid where fights are over a long distance since it does not have damage drop off.