I have a Samsung tablet and I sometimes use Google's voice command feature but have never experienced the kind of hostile takeover described by you. For example, if I tell Google to open an app or open a certain website, I can later do the same thing using the normal touch-screen technique with no issues. Likewise, when using my smartphone.
I think I would not use Google's voice command to try entering the password for a Wi-Fi connection. I say this because, if I tell Google to open a website where the website name has a hyphen in it, Google fails to recognize that character, whether I say "hyphen" or "dash". So, I suspect that trying to use Google's voice command tool where you need to enter upper case and lower case letters, numbers, and special characters, is not going to work.
I normally only use my tablet with one of two Wi-Fi connections; one of those is my router at home; the other is me using my smartphone as a Wi-Fi connection for the tablet when I'm away from home. So, I don't need to be entering passwords for Wi-Fi connections since I always only use these two connections.
If I did encounter the problem you've described, I think I might try visiting a Best Buy store and ask a Samsung representative (in the Best Buy store) about it.
EDIT: By the way, you might find a bluetooth keyboard an easier way to type characters with your tablet than using the touch screen since you have some vision limitations (which you've stated in some of your posts).