If I understand you right, you can access any amount of accounts on one computer, it's just all the world saves will be in the same file for all the accounts. If you saved all the worlds on that computer all the accounts will have access to all the worlds in there.
If you have two computers with a new world saved on each one then you will not have access to each others world. If you accessed one account from a strange computer then you won't be able to access your world saves from your other computer.
The worlds are saved to your computer, not the minecraft servers. You can still start another world but then when you get back to your other computer you won't be able to access that world.
you link it to your minecraft account
You don't connect it to Minecraft, you connect it to Game Center in order to get your account. Regardless of having an account or not, you can then use the seed codes provided in the app and use them in Minecraft to generate the worlds of your choice.
minecraft
Yes you can, their worlds wont appear on your computer, all you have to do is download the minecraft application. there is no limit on how many users are on one account.
Copy the minecraft worlds from the file explorer
The worlds are probably not stored on any official Mojang server, so are not linked with your account, they are most likely stored locally on your harddrive.
Minecraft
No.
yes.
No, only if you delete your .minecraft folder, minecraft isn't 'installed' so you can't uninstall it either, just deleting it from your desktop does the trick, all your worlds will be left alone.
Well there is one way to get into good worlds in Minecraft. Type in a seed.
minecraft worlds on PE are finite, and the water beyond that is infinite.