Hi Michael,
yes, first you should perform the steps mentioned above to get back a working system.
If I’m right, you only added Variables: mail/postfix/smtp/headerchecks to univention-mail-postfix.info. This entry is only required to make sure that upon a change of the UCR variable mail/postfix/smtp/headerchecks the file /etc/postfix/main.cf is automatically recreated from the UCR templates.
If the entry is missing, you have to perform an additional step for changing the value:
ucr set mail/postfix/smtp/headerchecks="foo bar baz"
ucr commit /etc/postfix/main.cf
invoke-rc.d postfix restart
Regarding changes for main.cf:
it is possible to add your own custom subtemplate for main.cf. This is described in the following thread. Please note that postfix is always using the last definition of an option within the main.cf. So if you redefine an option in your own subtemplate at the end of main.cf, postfix is using your value. There’s one drawback: postfix issues a warning when defining options a second/third/… time.