Yes, if SiYuan stops being developed, you can still get your notes exported as markdown files. Since SiYuan is open-source, you can also use the internal code to parse the JSON format notes.

It's open source, so yes, and you can export files to Markdown anyway.

Yes, your existing installations on your devices, whatever they are, can read your data as long as you use the correct Repo keys that are used to encrypt stuff. In the worst case, you can deploy the docker instance with older images to keep going on.