1. I am on a PC.
2. Yes, I have played through all of the routes of the base game without the DLC added.
3. I don't think so, it "downloads" and when the extract page shows up, it has the birthday DLC and Wyatt DLC as separate icons, and the icons look like a black page (similar to a Word Icon) with the firefox symbol in the middle. The only option to extract is when I right click on on of the icons and am shown an "extract all option", which loads a similar page without the extract red bar at the top. If I try to click on either one of them, it loads what I assume is a coding page on my current browser.