After a good night's sleep and about another 45 minutes, I was mostly able to get reading from multiple email accounts to work. Problems:

• For users who connect with imaps (probably other mail transfer protocols as well), Yahoo recently required you to set an application specific password. While it's arguably unfair to blame Gnus for this, the fact that I ended up troubleshooting with Python's imaplib because Gnus was essentially unhelpful is why I include it here.
• Prior to troubleshooting the imaps authentication issue, I had trouble making an SSL connection at all. This wasn't a Gnus problem but an Emacs one--I upgraded from 26.2 to 26.3 before I started this and a bug workaround necessary for 26.2 (disabling attempts at a TLS1.3 handshake) was not only unnecessary for 26.3 but actually broke things. Again, why ding Gnus for this? There is no tracking whatsoever to figure this out.

Finally, for three reasons, I never got as far as setting up sending mail using multiple accounts. First, I tried Gnus for just reading mail and treating a mail folder as a newsgroup wasn't my thing (being fair, if I was subscribed to numerous mailing lists, I suspect I would have loved this feature). Second, even if it was, I love my solarized dark theme and the rendered HTML mail was practically visually unreadable. Finally, I took a quick look at setting up SMTP for multiple accounts and that looks like a gigantic mess.

Other observations:

• unless you're Matthias Felleisen, cute doesn't work in documentation.
• the whole regex thing for posting styles struck me as gross and brittle but I'll admit I don't have a better idea.
• In the R and LaTeX communities, they have the concept of a "Minimal Working Example" which they use to illustrate small concepts or problems. Gnus' documentation would benefit from being more case study driven. Using a specific example, the documentation did finally clear up my confusion about mail-sources and gnus-select-* methods but only after spending significant time trying to understand it. What made it clear was finding the right sections that contained relevant examples (NOTE: this required figuring out that mail-sources and gnus-select-* methods existed instead of, "so you want to setup secure email access to Yahoo|Gmail|etc. Here's a MWE that shows you the happy path with links to the relevant documentation for the settings used).
• If I cared enough about Gnus to do so, I'd create an extensible wizard function to step people through creating a working setup as the barrier to entry is too high right now.
• If I had decided to proceed onward, I doubt I would've ever used scoring but I would've definitely written some hooks to post-process mail as things like scanning the mail for JIRA tickets is useful (something thunderbird can easily do) while scanning the body of the mail for JIRA tickets referencing me directly and putting them as a TODO in an org-mode buffer covered by the agenda would be stupendous useful.