Jeff Shenk, jaxjason, Yellowbox, Jerrold Green1 -- thank you all.
(I'm going to reply here to everyone 😉 .)
Good suggestions. Sad to report, I think I've hit a wall.
1. If one selects non-adjacent cells, one cannot format the borders (the "Select Borders of Selected Cells" tool's options are grayed out).
2. "Format➞Copy Style" _is_ unpredictable (by me) in whether it includes the border formatting in the style copied. It seems to run strictly by majority: if 50% of the borders are formatted one way, it uses it. Otherwise, it does not. Unpredictable tools are not helpful. I found no more on this in Numbers Help. I am not currently on a fast enough line to download the User Manual PDF.
3. "Format➞Paste Style" applies the copied style (whatever it is) to _every single cell selected_. I can find no way to apply the copied style to the _selection_ as a whole. If I select a group of cells with a thick red outside border, copy the style, then select another group of cells and paste the style, every single selected cell is given a thick red border. This functionality is useless for my needs. I want to apply a border format to the group of selected cells, not to each cell in the selection. Specifically, I want/need Numbers to recognize the outside border of a group of selected cells when using the "Paste Style" command. It does seem able to do this.
4. Table Styles are applied to entire tables. This, too, does not meet my need. I am not trying to format the whole table (any more than I want to format each cell) -- I want to format just the outside border of a group of selected cells.
5. @JerroldGreen: (Greetings from the Golden Triangle 🙂 ) I understand exactly. I'm using Numbers to do something unorthodox. I did not expect it to be the best tool for the job (but I did expect it to be a little better -- one other alternative is conditional formatting, but there are no border formats that can be used conditionally). In short, I'm already not using the program "correctly". Changing my workflow won't help me meet my current need.
Did I miss anything, or is this, indeed, a dead end? I'm happy to hear other suggestions. Thanks again for your help so far. It hasn't solved my problem, but it has clearly delineated it, which is very helpful.
--Kirby.