As KevinH pointed out, you are missing the xml namespace for the document:
Code:
<SetPasswordRequest xmlns="urn:zimbraAdmin">
As far as forwarding addresses, you can directly set the "zimbraMailForwardingAddress" attrs using ModifyAccount. There is a specific call for aliases because they are trickier, and require multiple updates in LDAP to reserve the DN of the alias, as well as update the alias entry on the account.