You don't need any designer. The templates generate complete HTML code so don't need any external design.

That said, if you are not familiar with HTML then having a visual designer handy is helpful for both learning HTML, and also constructing HTML snippets that you may want to embed in your site. Any visual designer can be used for this, as long as you can both design visually, and see the HTML underneath.

A slightly more complex answer to this question is that using browser development tools, like FireBug you will learn how to make global design changes by changing not the generated HTML but the CSS instead.

Cascading Style Sheets (CSS) are used to control the visual styles of the eventual web site. This gives you a lot of control over the final look of the site.

The answer to the question is "none". You don't need to know anything about CSS to get started. The CSS file that ships with NetTalk is a reasonably complete set of styles, that generate reasonably good looking web pages. There are a number of themes included to choose from.

At some point you will probably want to open the CSS file and take a look inside. Most of the settings are very simple, and can be easily changed with a text editor. 

Again the answer is, "no you don't", but again the longer answer is, "you might at some point."

JavaScript is the language that the Browser understands. Many of the features that NetTalk offers you are implemented using JavaScript (although you don't need to know JavaScript to use these features.) It's likely that many more features will be added as time goes on, also using JavaScript. However you will never need to learn JavaScript in order to create your own sites or applications.

You may want to add your own features, and to do this a knowledge of JavaScript may come in handy. However this is something you can learn when you need to.

No. XML is used under-the-skin but it is not something you need to worry about.

No.

AJAX is not a particularly well defined term, and has come to mean different things to different people. Originally it stood for Asynchronous JavaScript and XML. Later on it was used as a blanket term to include a number of different techniques which made web sites more responsive, and more usable. These techniques are sometimes also collectively known as Web 2.0.

NetTalk does make use of a number of AJAX techniques, including asynchronous JavaScript , but this happens far away from what you work on. The browses and forms (and other bits) are dynamic, and the interface for the user is very rich, but you do not need to be concerned about the mechanics of how this happens.

No. The Clarion program you create (the EXE) has the web server built in. There are no external dependencies.  One of the big goals with NetTalk Web Server was to make deployment of the server super simple.

Yes, SSL is fully supported.

Yes, all "modern" smart-phone devices are able to access, and use NetTalk sites either via their built in browser, or via the Opera browser compatible with that device.

Yes, creating SOAP servers, ie creating a Web services Server is really very simple. NetTalk includes procedures to create Web Services, and Web Service Methods. The application will generate a WSDL file for you so that others can easily consume your service. The services are also fully self-documenting which again makes it easier for other developers to make use of your service.

No, NetTalk apps are completely royalty free. (You obviously need a server to deploy the application onto, but you don't pay CapeSoft per-server or anything like that.)

Check out the handy guide Selecting a Tool for building web applications.