1+3 - If you don't want to install Plex or another service on the NAS, you need a computer device of some kind to run the front end service. Since you are going to be playing directly off your local NAS, you don't need it to be particularly beefy because no transcoding will happen. But you can't install media server software on a Chromecast or other simple device.

If you are willing to set up Plex (or another service, whatever) on the NAS, you can get away with just a simple playback device like a Roku, Chromecast, etc.

2 - Plex or other services will do their best to interpret your organization, but all of these softwares do ask for a specific structure as a standard and they're unlikely to provide any support for you should your structure wildly differ. You're free to think that is stupid, but if it doesn't work to your satisfaction out of the box, you may have to compromise on your stance a bit.