Malte,
Before answering some of your questions, let me say that theme-zen is still under (heavy) development, especially the part regarding install, upgrade and integration with acs-core (first round for integration has been committed few hours ago and has issues).
a) AFAIK, no, it shouldn't be changed. The dotlrn-master-custom is the one which allow switching among sloan, selva and zen themes at user and community level.
b) "Add new site template" and "Admin Parameters for Site Templating" buttons should appear above the list. If it's not the case, we have a new issue here.
c) theme-zen will be the default theme on install and we'll probably force it on upgrade too unless we can get help to work on CSS for sloan and selva themes (to be discussed at .lrn meeting today).
d) hmm? it should support both. Maybe it has something to do with the integration. We'll check.
To have theme-zen as default template, use the "Site templates"-"Admin params for site templating" from dotlrn admin but if the button doesn't appear on your install, well...
e) show/hide portlet buttons should appear in theme-zen. Not sure about the other themes.
f) Not sure what you mean.
g) see c)
h) yes, it should.
i) yes, theme-selva is installed by default.