I'll try to answer what I can.
RE: windows, yes it should work just fine, it's an interpreted language and therefor system neutral. The setup is a bit different, but functionally the same.
RE: local testing, that's the proper way to do things, learn and test things on your own stuff before trying to make things work in the wild.
RE: keychain, if what you want to do is make user facing things, python is probably not the answer. There are tools to make ui/guis and stuff, but most of the stuff is going to be command line or terminal interface.
You have come to a very important question you have to ask yourself if you want to learn to program, There is really 3 options.
- Frontend - User interfaces, webpages, things that the user interacts with, e.g. with keychain, which you would want to learn javascript/typescript and html / css etc.
- Backend - Stuff the user never sees, automation, bots, apis, servers e.g. this is the kind of thing nectar is really for, and by extension python in general.
- Fullstack - These are apps that do both, front and back and communicates with itself, single page applications, this is where a lot of the js frameworks fit, react, svelte, next.js, nuxt, vue, angular etc. It does have the added benefit of using user facing tools like keychain for the user, but I find it complex and hard to learn really.
Whichever route you chose, I'll be more than happy to try to lend a hand where I can.