I'm not sure if I understand exactly what you're confused about so, if this is off track, steer me back in the right direction.
Basic Economy (BE) is a fare type (N Class). It isn't a type of seat or section on the airplane. It gives you a lower price in exchange of your accepting additional restrictions. BE is not necessarily available on every flight, all the time. BE fares can be filtered out, when searching for a ticket on the website or on the app, so you might not even see them.
That drop-down box in your screenshot is showing class of service, not fare type. If BE is available on the flights in your search results, it'll show up as an additional column. If it's not available, or you have it deselected (selected should be the default), then that column will not be there.
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If you select a BE fare, the dialog box shown in post 155 will come up requiring you to verify that the BE restrictions work for you before you can purchase that ticket.
Positive bag matching ended for domestic flights some time ago. I can't seem to find the data with Google but I think it was around 2009 which was after your flight. Even so, when the rule was in affect, the bags could travel without the passenger as long as it wasn't the passenger's action which caused the separation. As long as you didn't volunteer for the bump, there was no requirement to remove your bag.
Also, the miles no longer expire. Thankfully, that rule went away, too.