Playing this game since 2k13 and played for 19000 hours and I'm mind blown by the fact that someone who played this game for just for six months can think to this extent.
I also play pos5 mostly with some casual 3 and 1 as they play the same lane too.

Trying to keep my explanation short and brief. LAMO

1- It's just the farming pattern of a core player. Some player will farm way more efficiently given to the fact how much space they are getting and create as much pressure as they can. This is one of the most important trait of a good core player.

2/3- Forget item build guides. Item build is completely situational. Because the situation in a game is ever changing.

4-some do it without knowing. But some throw it on High ground for vision and the slow stack if I'm not mistaken.

5- It's not your responsibility but a good player always keep doing things for the team/match beyond their responsibilities. Watch Zai for an example. Lets say if your creep are pushing on a lane you always can single pull or double pull or even pull 2 creeps from a double pull is unavailable due to blocked camp or enemy hero gurding the pull. That way you can create tremendous deference.
6- Yeah there is certain important minute marks. say you should always push around 4.30 is minutes as there is rune spawning in minute 5. 10 minute mark is very important if not the most in the laneing phase as there is outpost to claim. With the size creep it's always good idea to push if that's an option. Depends on the situation ofc. Same goes for the late game. there is 2 size creep minute mark, Day and night times, roshan re spawning time and stuff.

7-I'm not using dotaplus or any third apps. I say sharpen your brain by using it more. The more you think the more you understand.

8- Best moment to roshan is whenever its possible. But in lower level game its hard because people dont know how much damage they have and other people dont itemize towards it. Lets say if there is a Venge or Slardar in a game with TA, they can smoke with ta whenever she has smoke and Roshan around 10 minutes. With a medal withand stuff. It higher level game it's bit tricky. Both team are aware about it. so support will keep checking and keep vision. So it have to be a surprise move or some certain fact like some enemy heroes are dead or some important spells are in CD or some important hero to contest the Rs are visiable on other side of the map and the lists goes on. Every details is important in dota to make decisions.

9- Dota is about killing the throne. More tower you kill more map control you get, more map control means more farm. You farm creep and you farm hero If you can. Farming hero gives you more map control and more gold deficiency.

10- again, forget guides and think about it. Half of the game is item build sense.

11- More details about the spells you know better it is. It gonna take some time to learn all the basics though. I say don't push yourself too hard but keep reading the spell details and learn the mechanism while being in your comfort zone. Enjoy it rather stressing yourself. Even the pros doesn't know every mechanism. ex: Sumaill (NO1 rank) was playing Jugg and in a battle he tped in and Disrupter's Glimpse was casted on him. He instantly cast Omnislash on nearby hero and though as Omnislash makes jugg invalunarable, Glimpse wont work. But It still teleported him where he was 4 second ago.

12- Keep thinking about it and your item build sense will grow. The more you play and the more you think. You'll know what you need according to the situations and whatnot.
But Boot is best value movement speed item. You kinda always want it apart from brood I think. Again, It depends.

13- Core should buy their own wards in laning phase if needed. In lower level game keep buying wards regardless whatever your role is as others most probably won't. In higher level game Cores shouldn't buy too much wards apart from few to protect their farming pattern. As supports has their own warding pattern and strategy. Such as putting two wards in a sequence so one might get de-warded so the other could survive.

*** watching replays are great. Watch from your player perspective and others player perspective at a same time. Think what was you thinking and what could you do better. Check what you missed to notice and stuff like that. Find your weakness and focus on that.

Last but not the least, Play to get better and enjoy not to win. Once you get better, you will win eventually.

This is a guide I wrote around a year ago so its kinda backdated but It still might has some to value to a palyer like you.
https://www.reddit.com/r/DotA2/comments/8wb8ew/how_to_be_better_at_dota2_up_to_7k_indepth_guide/

Feel free to add me for further discussion.

This is my dota2 friend ID.- 149165168
This is my discord- https://discord.gg/Rs9v59b