Hello,
1 & 2) If you are hosting the odoo inside the odoo.sh platform then you can install the third party application, and also you can use the custom code with help of the server actions and automated actions, you can edit the base code by editor but it's temporary.
If you are using the online odoo then you can't allow to use the third party application, and also you can use the custom code with help of the server actions and automated actions, and user not able to change the base code.
3) If you self hosting the odoo then you can also use the third party application and also you can use the custom code with help of the server actions and automated actions, and user can able to change the base code but update the base code is not a good idea.
If you want to use the enterprise edition then you have to select the odoo.sh platform.
If you need more details or help contact us.
Thanks,
shivoham.odoo@gmail.com