This is quite a lot of work. I don’t think you can code that in a reasonable amount of time.
You could use a multi-site setup and a plugin to map single posts and pages as language alternatives to each other. The plugin Multilingual Press does exactly that and all the other points you listed. The pro version allows you to edit all language variants on one page.
And when you disable the plugin everything will still work; so there is no risk trying it.
Disclosure: I am now a coauthor for that plugin.
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