I started this thread, and I am afraid I spoke too soon. Relocating the two indicated files did allow TurboCAD (2018 and 2016) to start, but it is causing another weird problem.
TurboCad 2018 keeps trying to reinstall itself - sometimes when I just click on a folder, but most consistently when I start my .pdf printer, pdfcreator. I can generally get out of it by hitting cancel anywhere from one to ten times. Does anyone know why this is happening and how I can avoid it? I tried moving and renaming the TurboCad 2018.msi file, but it did not change things.
Thanks to all for their time.
Just tried deleting only TCWelcomeSplashTest.exe, rather than both TCWelcomeSplashTest.exe AND dbautosrvwrapper.exe, and (tentatively) this seems to be working, i.e. Tcad starts and attempted reinstalls have ceased.