When you first log in on the offline application it must gather information about the current user, which is required to make certificates while offline.
A mobile device installation can only be linked to one user. To change the user, delete the device in the devices section in the account manager.
Yes you can from the devices section in the account manager. Clicking on the lost device and removing it will restore the certificates to a state before they were downloaded.
Please contact support who will help check the compatibility for you.
If you are using the website then all the data is from the cloud. If you are using the application; clients, locations and certificates will show a cloud icon on their list entry.