Yes. When you run the newly installed copy of Joplin it will create a joplin-desktop folder and you will see the default "Welcome Notes".
Shut down the new copy of Joplin (CRTL+Q or File > Quit). Delete the new ~/.config/joplin-desktop folder. Copy the old one into ~/.config/
No, you don't need it. But if you have added lots of words to your spellcheck dictionary you may want to copy just the file Custom Dictionary.txt out of there and put it in the same folder for the new install. In case you later see it, there is also a folder ~/.joplin. You do not have to worry about that one, it's just where the AppImage is stored.
It depends on how skilled the fool is at being a fool
As long as the data is not currently corrupted or gets corrupted during the copy process you should be fine. Often, to test things, I shut down Joplin and rename my joplin-desktop folder. I then restart Joplin to get a default install with the welcome notes. When I have finished I shut down Joplin, delete the new joplin-desktop folder and rename my original one back to joplin-desktop. Joplin has always started up intact with the notes as they were before I started. This is basically what you are doing; you are just replacing the data folder.
Don't delete the backup until you are certain that the new copy of Joplin is running OK.