to answer your questions (as best i can):
as far as i know, you can not mix the syntax of one scripting language with another inside the same script, i.e., vbscript and jscript.. the closest thing you may be able to do (and i haven't looked into this yet) is to call a jscript function from a vbscript function, this is possible in the world of ActiveX scripting, i just don't know how XSI evaluates the scripting runtime and namespace and whethere they even evaluate the scope of more than one script at a time..
you can write different scripts using either of them (or perlscript and python, although vbscript and jscript lend themselves better to the world of COM which is what you're dealing with here..)
now, because you are using vbscript or jscript within an ActiveX scripting host, you can write your own compiled binaries and expose their functionality via COM interfaces (dual interfaces ala IDispatch will allow late bound languages like vbscript and jscript). this means that you
can write a COM component in VB, Java (VJ++98), or C++ (VC++6) and then call those components from within your script in XSI.. as far as i know (and i haven't tried anything yet), there's no way to expose the XSI object model to your compiled binaries (unless you performed some script site COM trickery using C++ but that would be ill-fangled and a waiste of time).. what you can do is access the data you need from within your XSI vbscript/jscript, and then instantiate a compiled COM component (C++ for speed and elegance) and then just make calls to your component by passing the data from the script to the component.. this is how a lot of distributed web apps are built.. ASP calling COM components. there's no need to do conversion of the data from script to component, because all data will be of type VARIANT or SAFEARRAYs and the default marshalling code will do this for you.. if you are not hip with the internals of COM, no worries, just use VB and you'll never know the difference..
to q#3, yes, the commands are the same in any of the scripting languages, you might call them with a different syntax but the commands name's and parameters don't change, nor do their ret vals...
for q#4, if you mean can you customize the interface? yes, within XSI and i believe you could write script to dynamically change the interface, although i'm not sure.. there is a way to run the app via command line and hide interface toolbars, etc.. any of this type of stuff would be done with the scripting, not VisualBasic itself..
i'm not sure i understand #5,
#6 is yes, see above...
#7, the benefit of multiple languages is flexibility.. the idea is why force users to learn yet another language MEL, maxscript, and now XSI script.. just expose a common object model to the application, and use ActiveX scripting.. now any ActiveX compliant scripting language can be used.. if you wanted to, you could write your own ActiveX scripting language (see ttp://msdn.microsoft.com/scripting//scripting).
its very flexible.. the reality is that if you understand vbscript, jscript is not that much of a leap, it is slightly more powerful when it comes to objects..
hope that helps...
benjamin