I tried the light test, the the pattern they showed was very much like your Morbid Medicine photo, and the cards didn't seem to show much of any difference in the amount of light they let pass through, though it's a little hard to compare when the pattern is different. The cards in question are Soulswap and Yuliana.

Oh, is this what you wonder. DM 01-09 has bit different card stock compared to DM 10-12. Surface of the DM 01-09 cards is more rough / has that texture while DM 10-12 has more flat surface.

Sure, but does the card stock quality also fluctuate more in later sets? Because my whole problem is the cards feel flimsy, not as rigid as other cards of the same respective set. The seller gave me an explanation about them losing their moisture in low-humidity storage space and so becoming more frail and/or thinner. Is that something you've encountered? I have no tools to weigh or accurately measure them, so I'm at a loss.

They seem real based on what I see. I sent you private message about better details

Im sure that those cards are real deal

Hey guys!

A few days ago arrived the Kellog's promo packs that i bought and just to be sure about their autenticity i opened one and now i'm more unsure of it ). I guess this kind of promos are not foils but still i want to be certain about it. So if you know how these promos should look like please take an eye at it and tell me what you think about them. Also i noticed that my older cards started to bend mostly the foils and they got a little bit darker as far as i can tell. Is this normal or the humidity is responsible. They were kept in slevees and still got bent, i had other cards that were laying around in a plastic bag and didn't get affected. Please tell me your opinion about them.

Hi @TJ! Kellogg's promos are not supposed to be foils. Yours look authentic to me. You can compare yours to the ones on the DM Wikia in the below link:
https://duelmasters.fandom.com/wiki/English_Promotional_Cards_Gallery#Kellogg.27s_Promos

You are correct about humidity being the cause of the card bending like that. Consider storing your foils differently to avoid this. I suggest you double-sleeve foils to minimize the amount of humidity that comes in contact with them.

Thank you for your response @Haiken, appreciate it. Should i put the foils in a place where they can dry out or just double-sleeve them and they will come back to the original form?

Double-sleeving foils will not un-bend them, but rather prevent them from  bending in the first place.

You could try drying bent foils. I haven't tried that myself, but there are videos out that claiming that it works. (Like this one: https://www.youtube.com/watch?time_continue=44&v=y-VTTdFH6Jg)
I suggest you go about it carefully, keeping a constant watch and start with the less valuable ones first.

If you attempt to un-bend them, do let me know how it goes. ^^

Thanks again for the tips on the double-sleeving matter @Haiken. I bough KC sleeves and double-sleeved the important cards(did not try to unbend them), now i saw in a video where it was explained that if they are stored in an album helps in maintaining the quality of the cards. Now, does it matter what kind of brand are the sheets? If you can could you suggest some brands or just send some links with the good products?

Happy to help. ^^

Well, putting the cards in sheets in albums won't make much of a difference when it comes to maintaining a card's quality, if you have already double-sleeved them. But it will help you keep track of them, make it harder to lose them or have them end up in bad situations (washing machine for example), and it will protect them from UV light (though any box with a lid will do). The biggest reason to have double-sleeved cards in a sheet is so that the sleeves themselves will be protected. That way the sleeves won't get scratched/dirtied as easily, and you don't have to replace the sleeves, which is a situation where the card can be damaged. Not having to replace the sleeves also minimizes the amount of humidity that comes into contact with the card.

Since you have double-sleeved the cards, the type of sheet doesn't matter too much. The cards won't get into contact with the sheet. I do however suggest you get sheets that aren't too loose. That way the cards won't fall out.

I personally use Ultra Pro Platinum sheets for my most important binders. (Not all cards in those binders are sleeved, so having acid-free sheets is a requirement for me.) For leftover cards, I have Ultra Pro Silver sheets. These are also acid-free sheets, but tend to be more loose and not look as good.