I know next to nothing about widevine decryption so forgive my ignorance - I was wondering if I download protected files now and get a widevine key later from another device, would I be able to decrypt the file with that key? or must the decryption be done with the same key/device that was used to download the file?
Yes. Depends on the platform. For example with Netflix L1 is required for full HD, L3 will only give you access to SD content.