If you have a DND and you’re not seeing any notifications just double tap your screen then swipe up and you’ll see all your notifications from the time DND was turned on. You won’t hear any sounds and haptics though

My opinion differs from everyone’s here but, I had my 8 Plus for 6 years with cheap cases from eBay, mostly for the looks. They ended up offering better protection than my Apple ones so I would suggest you pick up a cheaper one and replace it over two years than getting a 50€ case

You cannot. I was in distress the first few days since I thought that my phone was bugged and ended calls I have not made (the notification sound kinda sounds like a call ending)

In most of the apps (including photos) if you tap at the upper portion of the screen, it takes you to the top. Maybe thats what happening