This is a bit to try and tackle in a single post so I will respond to these in the order you listed them.
When it comes to having multiple items added to the cart with a single add to cart button we have an older How To on this that was created for eCart 3. The screen shots and some of the details might not match up but I think it is a good doc to illustrate the process that you must go through to get this worked out. Here is a link to it:
Please check on this and review the doc to get an idea about this type of setup. Please post back with any questions that you have about it and I will help you with what I can.