How to archive and backup our questions to our own PC just in case? In order to be cautious for possible future closing of the site....
At a low level of activity I would manually "Save as..." each Webpage containing your posts (using your favorite browser), producing a complete copy of HTML and images locally on your own PC. (I can also recommend some kind of backup strategy for your PC, of course.) Such copying is naturally allowed by the terms of Creative Commons by-sa 3.0 with attribution required.
If there were a larger body of work to save, scaling up with some sort of browser automation will be attractive. This would involve a substantial programming aspect, but there are lots of "web-bot" examples documented on the Web itself. E.g. do a search for "ruby web bot" or similar depending on your level of coding skill and interest.
In any event starting with a manual regime will give you an idea of what is useful.