Ok, I found a partial answer to this problem.
You can stop the "HKLM\Comm\InternetSharing\Settings\ForceCellConne ction\" key from re-appearing (or reverting back to the default value) if you run DotFRED's Task Manager, move to the Services tab, and press and hold "HTC CM Guardian" for a pop-up menu of options. In the menu, choose "Set Manual". Soft-reset the phone and the key will stop reverting.
Note that if you don't want to have to keep choosing "Sprint" in the "Network Connection" drop-down in Internet Sharing, you are free to change the value of "ForceCellConnection" to "Sprint", instead of deleting it.
However, this does not stop the "Error 67" (Incorrect username/password) error from appearing. So far the only way I have found to prevent it is to open Pocket Internet Explorer (or initiate a data connection some other way) before clicking 'Connect' in Internet Sharing. I thought clearing the HKLM\Comm\InternetSharing\Extension\" key would do the trick, but apparently not.
Any ideas on solving this last part of the problem are welcome.
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