No, they would have different IP addresses. If you want the new target server to use the same IP address; once the migration of the data is complete, you would shutdown the migrated server and then change the IP address of the new target server to the desired IP Address.
Maybe, depends how you define the usernames and passwords are on the migrated system and if NethServer allows you to follow the same rules for the usernames and password. If the rules on both systems are the same, then there should be little reason why you can't keep them the same.
I'm not sure on this one, I do suspect that you would need to setup the MS AD Authentication first as the setup may very well trash whatever user credentials have been setup in order to configure the MS AD Authentication. Not to mention the differing file/folder permissions in the users Home Folders as well as for other services as well.