There is no problem with using micro-SD cards in adapters. Canon has not recommended either using or not using them. Their requirements are that they be Class 6 (I recommend Cl-10) or higher. The camera doesn't care if the recording medium is Standard, Mini, or Micro as they all behave the same and are all built to the same standards. Canon is a member of the SD Association that sets the standards.
The one thing you do not want to do is be changing your card. Even though it is quite often offered as an option for people, the most common cause of SD card failure is mechanical. Usually the connectors will fail after repeated insertions and removals when there is a failure. However, if you need to change your card, then go ahead, just be careful and use a SD card case. Leave any Mini SD cards in their holder. My suggestion would be to just use a 32 or even 64GB* card and leave it. Use a USB cable to d/l the card when ever possible,
Since you have tried several cards all to the same effect, I would suggest that it ain't gunna be the card(s)**. Look inside the card slot with a small light or use a mirror and check for any debris. (NOTE: debris can only get into the card holder if you change your cards.) Also look to see if any of the contacts are loose or damaged. If you see any debris then use a blower to remove it. If there is any obvious (not too likely, but sometimes you can see damage) bad contacts then that is your problem.
If that is the problem or you still don't know, you will need to decide if you want to pay for the repair or scrap the camera. Unless you are still covered by the warranty, it can be expensive. The T6i is a good camera and probably worth having it repaired but I don't like telling people how to spend their money.
* I have heard some complaints about 64+GB cards as they use a different formating than 32GB cards. I haven't seen any myself as I use 32GBs in my cameras.
** It wouldn't hurt to look at the cards while you have them out. Check the contacts for bent, warped, loose, or even missing ones. The Standard size cards have 9 contacts and the Micro has 8. One of the Std contacts is reserved for future applications and does nothing.
Sorry I can't help more than that.
All I can say is that in the 3+ years that I've been following this forum, we've fielded enough complaints that turned out to be solved by stopping the use of micro-SD cards to convince many of us that it isn't a smart idea. And if you believe that the contacts are a memory card's weak spot, why use a setup that has two sets of contacts instead of one?
Tell you what, Mr_Fusion. You call Canon at 1-800-OK CANON and ask them whether it is OK to use micro cards with adapters in their Cameras.
If they say it is OK , I will always answer the question with "I don't think it is a good idea, but Canon and Mr_Fusion must know more than me, because they say it is OK."
The one thing you do not want to do is be changing your card. Even though it is quite often offered as an option for people, the most common cause of SD card failure is mechanical. Usually the connectors will fail after repeated insertions and removals when there is a failure.
Sorry I can't help more than that.
The most common cause of failure is mechanical, I agree. That’s the best argument against using micro-SD cards. But, do not reply to observation. Please.
Your claim that Canon does not say you cannot use micro-SD cards, is too hard to argue against.