Many questions about the GAME The Witcher 3 :).

Hello i am new here:),I have started playing TW3 a month ago, spent lot of time and decided to stop bit to get real life things done, because i could not stop playing. I decided to come back and i have few questions, i hope hardcore fans could find the answer, again its game related and i am in my first run.

1. Side questions : Does it have any impact or reward if you are able to do ALL the quests ? I mean "ALL" including the ones you find randomly on the streets of Novigrad etc..
2. I saw you can change the difficulty of the game in the options, does it have any impact to change the game difficulty ? Does it have impact in game if you change Gwent difficulty in the option?
3. I watched old reviews of TW3 during the time i put the game aside, and i saw in the past it was possible to equip only 1 or 2 potions at the same time..How and when did this change occure? (We can equip up to 4 potions now). And how many other changes have been made, which ones are the most popular? (I am guessing only full hardcore fans could answer this question)
4. MOST IMPORTANT QUESTION : Just before pausing playing the game, i remember being level 20'ish and trying to fight some big bird monster (not far from nilfgadien ruins, near a swamp i think) it was 30'is+ level and it had the red skull indicating this monster is way too powerful for me.Well, with the combinations of the awesome concotions i had made and the use of oils, i was able to kill it. And BIG DISSPOINTEMENT, the treasure i found was worst than what i found in monsters 15 levels below. I went googling and saw that monsters/rewards were "scaled" to our level, something like that?My question : How is it scaled? At which level it is best to tank a monster? Does someone has the details and precise informations about this, or just a really good broad idea? I tried to search and could not find.To be honest i like challenges, and like to beat monsters that outleagues me. So i was bummed out to hear about the exsistence of a "items/treasures scaled to your level" thing.
5. Are there challenges, players came up with? Like beating the game without the use of magic or anything of the sort?
6. Is there any new exciting patch/content planned in the futur, still?

Thanks

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Btw, the musics are so good, i saw on youtube, that few musics were not even in the official OST ( I am thinking about "Outskirts of Novigrad") . Thx again