Much if not most of the OH 1.x documentation is still valid for OH 2. In particular, Items, Sitemaps, Persistence, Rules, and configuration of all 1.9 version bindings are all the same as documented in the OH 1.x wiki.
Probably but it depends on what you really want to do. If all you are using is zwave and all you want is a dashboard (i.e. no automation) then you probably don't need Desginer and can get by using PaperUI and Habmin, though Designer will make it easier to create Sitemap files for ClassicUI and BasicUI.
Make sure the device is done being configured. In PaperUI or Habmin the smoke sensor should have make and model information associated with it. If it doesn't and/or it says "unknown device" and there are not Channels listed you probably need to wake the smoke sensor up one or more times so the zwave binding can interrogate it to figure out what it is and complete configuration.
Once it works and you start getting values (- means the Item has not yet received any value) you will want to set up persistence so you can chart the data. If you want a lot of control over your charts I recommend:
Habpanel is a nice alternative that doesn't require to you create text files to configure your dashboard. Habmin also lets you set up charts and graphs without editing text files.
The question doesn't really make sense. openHAB 2 is like a platform. You can run add-ons on top of this platform to do things. One set of such add-ons are UIs and each UI has different capabilities. For example, you can do a whole lot using PaperUI including managing add-ons, working with Items and Things but you cannot create sitemaps, you cannot configure 1.9 version add-ons or create Items that work with 1.9 version bindings, and you cannot create rules. Habmin has more capabilities including the ability to set configuration parameters on zwave devices and create Rules, but it can't create sitemaps and I don't think it can do a whole lot with 1.9 version bindings. Habpanel, ClassicUI, and BasicUI are all intended to be the user interfaces one uses to control the home automation, not administer the deployment.
Ultimately, it is hard if not impossible to make a blanket statement about what you can do through the UIs verses the text files. Almost every deployment I'm aware of ends up using both.
And if you have any text files I recommend Designer.