Lee Gummerman (of Symantec) graciously replied to my request for information on the problems installing the SAM 4.5.1 Update:
"I think your recent update covers some essential points but. after talking to our support people, I wanted to pass on some other information.
First, you were essentially correct about the 3 versions of the patcher and their correct use. One thing to note is that the Fat patch can be universally applied to any version, the resulting 4.5.1 version being Fat regardless of the original 4.5 version (Fat, 68K or PPC). Unless the user custom-installed a 68K or PPC version of 4.5 (the default Easy Install yields a Fat version) and wants to keep it that way, we recommend using the Fat patch.
The new information I received is that in some cases, either due to corruption or other modification of the original files, the patcher may not correctly recognize a 4.5 module. In this case it is best for the user to reinstall 4.5 from their original disks (note: this does NOT overwrite the SAM Preferences file, so the user's current settings are preserved) before applying the patch. Our Technical Support has found that, except in some very rare cases, this solves any patcher problems. In those unusual cases, the user can receive 4.5.1 disks through Tech Support.
I am adding a line to the patcher Read Me files and splash screens which will advise users having problems to do a clean re-install. We will continue to explore why modules might not be recognized so that we can possibly update the patcher at some future time to handle more cases without a re-install, but I can't currently promise any specific improvements until it is researched further."