Hi BGH welcome to gamedev!
Many of the questions you asked have already been answered many many times before so I recommend you try the search features of the forum, however here is a brief summary of the answers you'll find:
- XNA is no longer supported correct, however it works and will continue to work for years to come. So if you are new to coding games, stick with XNA until you are happy to move on.
- MonoGame would be the next stage *after* building your game, it's very straight forward (not easy) to port to as long as the game is optimized and does not rely on windows only features. Also as they are still working on the content pipeline, it's very useful to still have XNA installed.
- XNA doesn't actually have any plugin's, you can get 3rd party libraries that give you extra features, but no plugins that directly tamper with the API. Which 3rd party libraries you are planning on using? (note: most of the popular ones like physics engines etc... provide a DLL that works on MonoGame too now days).
Try not to dwell on trying to port before your game is built, it's a mistake I and others have made in the past over and over, this tends to lead to games never getting built, but that's just my two cents