Condolences. I'm not on Windows, so I can only offer general guidance.
First, can you confirm that
'C:\Users\David Stevens\AppData\Local\Programs\MiKTeX 2.9\miktex\bin\x64\pdflatex.exe
exists, that it is executable freely, with whatever permissions RStudio is running under and, if possible that from powershell or whatever the current CLI that W10 uses that pdflatex.exe anything` responds?
And, as the cover of Hitchhiker's Guide to the Galaxy admonishes, don't panic! Worse come to worst, there's always the option of doing the work on RCloud instances or a provisioned EC2 instance. (Or asking friends for help!)
Let me know what you find. I'll be online for a couple of hours.