3. If the address associated with the account changes
If the address associated with the account changes, the effect will depend on where the addresses are routed. If they are both to Nexus, the only effect will be on Calendering with ical/davmail which uses the email address rather than the username to make associations for sharing. See for example http://www.oucs.ox.ac.uk/nexus/calendar/iCal/davmail.xml where it refers to someones primary email address.
If the address on the account changes to one that is not routed to Nexus, then a forward will be automatically added. Even if you remove the forward it will be added back the next time the Nexus process runs!!