After 3.25 hours, I figured it out and finished my desired configuration.
I had to configure keybindings in two different, sensitive and careful ways in
1. ISB42 - Combat Broadcast, and
2. ISB42 - Combat Broadcast Hotkeys.
Even
MiRai's video from 2012 doesn't go through this process, because things changed, of course; but it's such a critical, make-it-or-break-it process for anyone who is not using the default 1, 2, 3, etc., keybindings on the main spell bar.
Anyway, I'm sure any smart user who encounters the same issue and reads the description for both of the above Key Maps (groups), will figure it out. As mentioned, it takes a long, long time to finish the process accurately for 2 spell bars, but one hopes you only have to do it once, and you're good to go with any multiboxing new group you form thereafter.
Thanks again for trying to help me, and I hope that the details (or walls of text, depending on your perspective) that I posted will also help you, as developers and admins, to help others faster. Maybe even improve the software so it's less complicated. I fully acknowledge that ISBoxer is super powerful, and that's where all this complexity stems from. But as Einstein said, genius is simple. I can guarantee that if you hire a "usability expert", maybe for cheap from a freelancing website or something, and he has the least amount of knowledge about WoW, or even MMOs generally, he will open your eyes to ways to simplify the program, even make it intuitive, while not sacrificing an iota of its power and configurability. And I can almost guarantee to you that if ISBoxer becomes 80% easier to use (and it can!), the number of people using it and loving it will soar, along with profits, naturally.
From my perspective as a user with some basic understanding of marketing, I can tell that ISBoxer has absolutely no other obstacle to explode with more success: just the degree of its user-friendliness, intuitive use, and ease of use. You have everything else...supportive developer(s) and moderators, lovely videos, an intricate Wiki, albeit in need of tiny updates here and there, and a Porsche engine of a piece of software. All that's left is how easy it is to use. An excellent book that explains the basics of what I'm talking about is
Don't Make Me Think, by Steve Krug.
Thank you again. And talk to you again about a smoother issue. =)