1: I know that someone was working on a WordPress Importer, not sure if it is done yet or not but it is in fact something that was in the short term plans.
2: You can use the Banner Plugin and insert that into the Blog Custom Header. This can be found at the bottom of Create/Edit Blog, its a large text area where you can customize the blog header. Also if your Banner is only 1 image. Then simply insert the image in there instead of activating the Banner feature.
3: There are many great TikiWiki sites out there but since you seem to be focusing on Blogs and Publishing in general here are two sites that use Tiki Blogs and Publishing very well