HI! try the ""revert to factory configuration" and see if that doesn't make your sapi 5 synthesizers work again using NVDA. I might have been ignored because mine it restored my sapi 4 engine, but I bet you'll have a good result if you give that a try before you attempt to re-install or repair all of those sapi5 engines. NVDA, has an issue where if you get into the control panel of it's synthesizers, it attempts to write to one of it's configuration files for a sapi engine and can't or gets interrupted in some way, it corrupts the file, it's own reference to it, so the "reset to factory configuration" somehow fixes it.
Try and see if it works for you. I bet you'll be surprised since it is working on the logon screen as you use to have happen, just as mine with sapi4.
I have an Eloquence set of voices from an installation of Kurzweil I use for sapi4 since I don't wish to pay for a sapi5 version of the Eloquence voices, and I saved a configuration which works on my logon screen. I attempted to have NVDA use a different voice under sapi4, and after changing speed and some other settings, no matter how often I press "ok," from within the NVDA dialog, it would not exit. So, when I re-booted to get my speech back it was a corrupted sapi4 engine that NVDA falsely reported, but naught was the case since every other screen reader which supported the sapi4 engine still worked fine so, hmm! I go, why?
If you re-install NVDA I suppose that fixes it too, but it is like using a hose when a squirt gun will do. :) So, try the "reset to factory configuration" first before re-installing all those sapi5 engines.
Let me know how you get on with it.
K 6 V F O