You're right. If something goes wrong, you can simply shut down UCS and start Zentyal again (in a certain time frame).
Correct. Also look for DNS records. A UCS master usually creates a SRV record domaincontrollermaster._tcp.
You should remove those to prevent any problems if you do the takeover a second time.
The "old" AD domain controller (Zentyal in your case, Microsoft in most other cases) is not demoted, you just simply shut it down. In the very last step of the takeover, UCS grabs all FSMO roles, adds a DNS alias with the name of the old AD domain controller that points to the UCS, adds the IP adress of the old AD domain controller as a virtual IP and removes the old AD domain controller from the samba database.