Why are the email settings fields not showing, and how do I fix this?
Since I was short on time I ended up reinstalling Craft. Not exactly a minimum effort solution, but it fixed the backend > settings > email problem (and the updates page, that was not functioning properly as well). However, new install would not show my plugin in backend > settings > plugins. The solution to this, was to create a bare minimum test plugin (just copy pasted from https://craftcms.com/docs/plugins/setting-things-up), after which both the test plugin and my real plugin showed up in backend. All seem ok now.