That's the cool thing with Threema. If you verifyed the contact is so prominent, everybody want to verify each others key, even people who don't understand the concept. In Threema it is not hidden somewhere, it's just allways visible on each single contact with green or red dots.

YES !

first it automatically trust the devices but if you want you can improve it with manual verification :

https://gultsch.de/trust.html

"TLDR: Automatically trust all new devices of contacts that haven’t been verified before, and prompt for manual confirmation each time a verified contact adds a new device."