A perfect solution for BrowserSim must match the following criteria:

• Be Java-based
• Allow to fully customize the application window [skinning]
• Work on every supported platform (Windows, Mac and Linux)
• Use browser already installed in the system [to avoid library conflicts and reduce the size of our plugins]
• Allow to execute JavaScript and/or modify DOM [to simulate some mobile * JavaScript events and Cordova API]
• Allow to change User-Agent header
• Allow to disable web security or support extensions [for Cordova cross-domain requests]
• Support debugging protocol or support extensions [for better LiveEdit/LiveReload]
• Support debugging protocol or ability to launch Web Inspector/Chrome Developer Tools [for JavaScript debugging capabilities]

Unfortunately, there is no perfect solution, at least I do not know any.
This is the list of the libraries I consider for BrowserSim (+=Pros, -=Cons, ?=questionable):

• Java wrapper for Chromium Embedded Framework
  • + I was able to run it on Windows
  • - It failed in a half of a minute
  • - Not supported since 2010
  • ? probably will work better with the most recent version of CEF
  • ? looks like there is no Linux support
• Qt WebKit bindings for SWT
  • + I was able successfully run on Windows (32-bit) with Qt 4.8 and MinGW
  • + At least during my smoketesting it did not fail
  • - The plugin page claims that only win32 and cocoa32 are supported
  • - requires Qt (and MSVC or MinGW for Windows) libraries to be bundled with it
  • - Does not have existing API for invoking JavaScript.
  • - Not supported since 2010
  • ? I do not know how easy it would be to run it on Linux
  • ? I do not know if it will work with Qt 5
• AppJS - a JavaScript SDK for creating Chromium based desktop applications
  • + Supports Windows, Mac and Linux!
  • + Allows to disable web security
  • + Has active community
  • + Did not fail at my smoketest
  • ? need to ensure that applications may be run without borders
• node-webkit - the idea is the same as with AppJS - allows to create desktop apps with JavaScript
  • + supported by Intel in some way
  • ? I did not try it yet
• Google Chrome Frame
  • + the only way to embed already installed Chrome into our application (all other require libraries to be bundled)
  • - Windows only
  • - no success to use it as Ole object
  • ? Probably will work if use it with Browser(SWT.NONE)
• Stay with existing SWT bindings for WebKit
  • + We already use it
  • + Support all platforms
  • + Allow JavaScript to be executed
  • + Do not require to be bundled with our plugins
  • + Allow skinning
  • - Require Safari on Windows [no updates since May 9, 2012]
  • - Web security cannot be disabled
  • - Do not support extensions
  • - Do not support remote debugging protocol
  • - Do not support Web Inspector/Chrome Developer Tools

To conclude:

• Firstly, moving to a JavaScript SDK potentially could resolve all our problems, but now I do not think it is worth it.
• Secondly, it would be great if we had good SWT bindings for Chromium, but they do not exist. The only way to get them is to implement them by ourself.
• Thirdly, the last option does not look too bad. And probably it does not worth it to create Chromium bindings or to move to another non-Java language. Especially, if we would be able to create a good workaround for cross-domain requests for Cordova, then almost everything else is ok.
• Fourthly, the biggest issue with existing SWT bindings is that Safari is not supported anymore on Windows. It is an OS specific issue and we could consider a replacement of SWT bindings on Windows only (e.g. use Chrome Frame or Qt on Windows).