Change my mind if I'm wrong

Well, really tell me if I'm wrong. I am not a pro but I have certain thoughts and I want to see if these thoughts are 'correct' or not. Anyway, let's start.

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1.  Simplifying games are 99% good under material advantage / in equal material without giving up too much positional advantage.

You're half right there. It's tough putting a number like "99%" but yes, in the vast majority of cases, if you have material advantage, simplifying the position and trading down should be beneficial to you. With equal material is very debatable. Sure, there are less chances of you being attacked, but that doesn't mean that you're going to win. You don't want to trade say an active knight for a bad bishop just for the sake of trading. Technically speaking, if material is equal the less pieces on the board, the higher chances of a draw... so it depends - among other factors - on whether your fear of losing is greater than your desire of winning.

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2. Middlegames and Endgames are more important than Openings to beginners.

Openings are not that important for beginners because chances are whatever advantage you or your opponent manage to get out of the opening will be blundered later in the middlegame or endgame. And that advantage is likely to be blundered as well. Some important chess player that I don't recall who said a long time a go that "The game is won by the person who makes the next to last mistake" . Endgames are important for beginners when it comes down to converting an advantage. Rarely beginners games will reach a drawn endgame and even more rarely is that both players will know how to play it properly. In most cases whenever beginners reach an endgame there's already an advantage to one of the sides. So I would say that for beginners, after the very basic endgame studies, the middlegame, especially tactics tactics tactics will decide most of the games. Actually, just not hanging pieces will decide most of the games. 

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3. Castling early is 99% better than not castling at an early stage, provided that the pawn structures are not damaged. 

Sort of.... 99% means 99 times out of 100. Once again, using percentages like this is not the best way to reach a conclusion. As a general rule of thumb, castling early should be better than not castling, but like every other concept in chess, it depends on the position. For example, if you castle too early, depending on your opponents position, you might be simply inviting an attack/pawn storm.

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4. In general, kingside castle is better than queenside castle. 

Your opponent can sacrifice a bishop for two pawns and an attack if you castle king side as well. That's not exclusive to queen side castling. An additional move to get your king safer to the b square may be a tempo loss, but sometimes in chess making a "waiting/safe/useful" move so that your opponent has to make a decision can be beneficial, but that might be a somewhat more advanced concept. Like pretty much every decision in chess, it depends on the position. It's hard to talk in general terms like this. But if your win rate is higher on kingside castles then maybe you're just more comfortable/familiar with positions where your king is castled kingside, and you are going to want to play positions that you're more familiar and comfortable with so that might be true to you..

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5. To prevent loss in opposite castling battles, I should take the initiative to attack the opponent king.

As a general rule absolutely. That doesn't mean you won't have to pay attention to the side you castled to prevent threats. But if you can have the initiative to pawnstorm and/or attack your opponents king you should definitely do that. Funnily enough, because it's chess, there will be a few instances where even though castles are opposite sides, your play will be on the side you castled, but that's an exception and a case by case scenario which shouldn't be your main worry.

As far as attacking it also depends. Your general idea looks sort of right, but it's chess so it depends on what your opponent does... going back with the knight to e2 (unless you need it for defensive purposes) looks wrong. You have a beautiful outpost on d5 for your knight that makes it extremely powerful (if your opponent leaves it there it can do a lot of damage and even potentially be sacrificed) which will force black to either give up a defensive piece to get rid of it or play c6, which weakens their king and creates a contact point or "hook". Contact points/hooks is something you should always be aware of on opposite side castling whether you're attacking or defending. If you can open lines in front of your opponent's king it's good for you. For example, you shouldn't let them close the position in front of their king. While pawnstorms tend to be good when attacking, you don't necessarily have to advance all your pawns. Sometimes it would be better to play a4 then Ra3, then Rb3 or Rc3 and have your rook not blocked by any pawn. Similarly if you're attacking on the kingside with h4 Rh3 Rg3... So once again, like most of the things here, it depends lol..... As a general rule, an attack tends to be succesful when there are more pieces attacking the king than there are pieces defending him

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6. If you have knights and your opponent have bishops, simplify it into an endgame, or vice versa.

Couldn't really follow what you meant but regardless of middlegame or endgame, in an open position the bishop pair is  superior. In a single bishop vs single knight endgame, as a general rule, the bishop is superior to the knight when there are pawns on both the king and the queenside, since it can move faster from one side of the board to another when compared to the knight. If all the pawns are on the same side of the board then the knight might be superior because the bishop is limited to one color. But it depends on others factors such as how close or open the position is as well. There are many many many exceptions to the generalizations you made in #6

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7. Castling when both sides have less than three pieces (without queens) is still meaningful.

The most important is to have fun (and/or get paid lol). After that the most important is checkmating your opponent. I get what you're trying to say when you don't want your opponent to develop with tempo, but if you were already planning on moving your king to a specific square it will not be the end of the world (or game) if you let your opponent give you a check, even if it means they get to develop a piece while doing so. I said it many times and I will say it again. It depends on the position.  Kings are very important in the endgame (some people give it a symbolical value of 4, which means more powerful than bishop or knight, less powerful than a rook, but it's chess, so it's a relative and it depends) so if your king is castled it might be too far away from the action, for example, defending an important pawn or escorting one to promotion. It's hard to explain without examples but if your king is not under heavy fire, castling will likely not be a priority when there are no queens on the board. But sometimes it can be. I think I might have said it depends on the position.

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Please give me an idea of these thoughts, whether there are flaws that are not negligible, or how these thoughts should be more deeply enforced in my mind. I would be very grateful if you could spend a few minutes answering one / two of my thoughts on chess. Trophies will be sent if you could provide useful answers or replies. Thank you for your time reading the post and thanks for your reply in advance!

It seems to me you're too focused on generalizations ( "3 pieces" , "99%" etc etc) and trying to play chess based on "general rules" instead of evaluating the position and calculating the best move. So while there is some truth to what you said, I tried "criticizing"  and "finding flaws" in what you said because chess is a game of exceptions and you can't rely only on concepts and general rules, so while concepts are important, they are nothing more than a basic guide to give you an IDEA on how to treat the position. So if you get too specific with it like "3 pieces" or "always trade bishops for knight in endgames" or "castle in move 5-10" it will only harm instead of help you. I don't really care for trophies, just thought I'd share my views on what you said because you seem very interested in learning and improving. Long story short, it always depends on the position. 1-5 are somewhat ok. forget about 6 and 7. 

Good luck with your chess and most importantly have fun

All power You have is on the table at beginning. You need use it wise way. All periods of the game are important. There is only one target, kill the king. If it is not possible next is draw. If it is not possible, lost with fight. As long as You have one, there is hope. Chess is not all in game. You can eat as much as possible, use time as much as possible, You can make harm and sell You life by as high price as possible. S U!,

Don’t simplify if opponent has low time

I don't quite get what you mean.