1. Look at the AJAX object for a way to download files from websites. I hear you can get more complicated communication if you make the website with PHP, but that's outside the scope of what I'm interested in.
2. You can either convert the text to an array with a for loop and using tokenat() and tokencount() kind of like in my capx, or you can directly add the commands to the array with the push action, one word at a time. After it's all in an array, you can use the array.asJSON expression to get a text representation of the array. In fact you can save a file with that to your website and use the array's download action and bypass the ajax object.
3. My capx was a simple example case of this. You'll have to add events to handle every function you want to use.
Overall the process to make an interpreter is much like doing it in any other programming language. It can get as advanced and complicated as you like.