Without opening them I don't think so. I tried that with one I bought out of curiosity and couldn't get my console or phone to detect the nfc tag. Yes they will work on more than one device if just using it for in game bonus items. But can only use it for one game save at a time. More handy info here https://www.nintendo.com/amiibo/faq/
It didn't in the website, amiibo wasn't listed as its own category when I purchased.
Clearly says amiibo is 1.05% cashback on the topcashback app
They're simple, just tap them on the controller. I'd say buy from the UK to be safe, stuff from China is often a knock-off in my experience, and the BOTW amiibo look great!
A waste of money to have a nice collectable figure? Because you can get a card instead? And you state that you understand the collection side of things :| What a pointless and annoying "contribution" to this thread.