I am wondering whether or not a Raspberry Pi (connected to the official Pi touch screen) can be used as a second monitor for my PC.
I think you should be able to achieve this with an Auvidea B101 HDMI to CSI-2 Bridge. I'm hesitating to recommend it wholeheartedly, as their website is a difficult beast to navigate and their documentation and software support is minimal. It would be a good idea to take a look through as many forum threads as you can find (in particular this thread on the raspberrypi.org forums) to assess the level of challenge you might face getting the thing to fire up.
That said, there's a plausible demonstration of exactly the setup you're looking for in the 'Demo 2/Laptop HDMI to Raspberry Pi 3' section of this video posted to Youtube by user Samir Sogay. It shows an HP laptop running Windows 10 connected via its HDMI output to the Auvidea B101 board, which is in turn connected to the Pi's CSI camera input connector. The laptop recognises the Auvidea board as a second display with maximum resolution of 1280x720 (the maximum resolution of the converter), and allows screen mirroring to it via the standard Windows 10 graphics properties window.
I think it's going to be difficult to find a solution that allows you to use it as a touchscreen while it's connected to a Windows machine this way. The touch signals aren't going to be conveyed over the HDMI cable, so you'd need to figure out some way (I'm unaware of any suitable existing solutions) to translate those to something usable by Windows and get them into that machine as a recognisable Human Interface Device. Tricky.