a core-prerequisite is nothing more (or less) than an include script (.sbi).

Once you know how to detect an existing Google Chrome browser installation (e.g. via a registry check) you can develop #A (case 1 and 2). A separate screen for prerequisites is no problem at all. You can use a standard dialog (e.g. Checkboxes) or a custom dialog to handle it. And of course, you can download the runtimes via the Download File script function.

Just send me a prototype of your separate screen (text you would like to see, etc.) and we can try to develop a sample project for you.