Yeah, I noticed that the option "local" is pretty much empty on piefed.ca I wonder if we create the same communities there and group them together would work.
Here is their repo, https://codeberg.org/rimu/pyfedi/issues I imagine you can propose some of those ideas there.
I think one of the problems on migrating users right now is how lemmy stores password compared to piefed. https://codeberg.org/rimu/pyfedi/issues/712 - But technically, it should work if you ask the user to write a new password.
You probably will not have access to this.
Only if you are an Admin. https://join-lemmy.org/docs/index.html
The data seems to be stored in a postgres database, and could technically be queried.
It depends on your need, if it is for legal purpose, it depends on your jurisdiction. Maybe web archive is enough for this case, https://archive.org/.
Otherwise, you can use the API as others mentioned. Or you can use Selenium/Scrapy/BeautifulSoup to scrape the website.
If you would rather not program something with code, look into browser add-ons that scrape websites, they are mostly visual, and you click on the things you want to save or navigate into. I am not familiar with them to recommend you something, but there are plenty of videos on how to use them.
That said, depending on the security of your instance, your ip/account might get flagged.
Talk with the Admins of your instance first and express your intentions, maybe they can help with what you need.