First, a big thanks to all who have responded so graciously and most importantly, respectfully! I see so many forums where noobs are castigated/embarrassed/etc. Perhaps some of those noobs deserve some of it, just as I did with my rant!
Second, a big apology for my rude, snarky, angry post. I really should have 'slept on it' for a few hours.
Fourthly, I'm not too good at math!
OK, the crash was my first with Sigil. Computers (even a Mac) can do that, usually at the worst times! Auto-saving is one way to negate the damage that usually causes.
It sounds like you need a basic tutorial though
Guilty as charged! "But I've been using Word for years! Why would I need to read anything about Photoshop?!" LOL! Actually, I have done quite a bit of web site creation (no awards, yet, but... ), so I tend to look at ePubs as just a sub-set of X/HTML. And I suspect it has many of the same "What you type may NOT be what you see" in different ereaders just as different browsers (especially those that tried to create their own "standards") don't always do things the same way.
Perhaps the most important information from your answers is to explain the very basic definition of an ePub file! That completely explains Sigil's behavior and file display. I will swear that I tried double-clicking on the files displayed in the Book Browser without getting one to appear in the Code View. Of course, now things work that way. My only explanation is that, somehow, while the machine was turned OFF last night, at least one of you "guys" broke into it and changed the code in Sigil! Well, it's slightly possible that I didn't actually do the double-clicking' part... but that wood bee meye furst mizteake uv thu yeer!!! So it must be you guys...
Now that I understand the function of Sigil better, I can see how it can be more useful. You are correct, it's not the best HTML editor but it's more than sufficient and it even has color-coding to help one see common errors. And the parser does a pretty good job of pointing out where an error might be. So far, I've taken the warning that the "Automatic" correction might send all your work into the bit box of Heaven as an incentive to make the corrections myself!
Thanks, again, for your kind and respectful help!