I am a network technician but we do mainly work with Macs now. That being said, I do have some experience with 8.1 and if I recall correctly (I still use Windows 7 as a daily driver on PCs) you can click and drag the lock screen with the mouse. On that screen I thought there was a switch user button after you slide the screen up.
We squashed most issues so at least I have been of some help. It is just perplexing on what this issue could possibly be as it is not a widespread issue with Windows 10 or Boot Camp. Furthermore, I know a few people running Windows 10 on their 09 and 10 MacBook Pros without issue and I am getting some of my information from their experience.
With Windows 10, the lock screen slides up and in the left-hand corner there is a list of all the registered users on the computer. The issue here all along has been that I can't slide the lock screen up with my trackpad, because it can't detect physical clicks.
It is a perplexing problem as I have not been able to reproduce it or anything similar on any PC/Mac other than my own. Thanks for sticking with me and helping through all this.