Interestingly, The Chicago Guide to Grammar, Usage, and Punctuation (Bryan A. Garner, 2016) offers Stevenson’s sentence as an example under “Using Semicolons”:
Use a semicolon to separate items in a list or series when (1) any
single element contains and internal comma, (2) the enumeration
follows a colon, or (3) the items are broken into subparagraphs.
- The drawing-room began to look empty: the baccarat was discontinued for lack of a banker; more than one person said goodnight of his own accord, and was suffered to depart without expostulation; and in the meanwhile Mr. Morris redoubled in agreeable attentions to those who stayed behind.
(Never mind that, 140 years later, it’s likely coincidental that Stevenson’s 1882 use of semicolons there fits both (1) and (2), given how variously they are used throughout his text.)
But let’s address your issue . . .
While it’s true that stylists proscribe the use of a comma in the second part of a compound predicate—that is, where the two verbs share the same subject—it’s also true that, as The Chicago Manual of Style (17th ed., University of Chicago Press, 2017) says under “Commas with parenthetical elements”:
If only a slight break is intended, commas may be used to set off a
parenthetical element inserted into a sentence as an explanation or
comment.
I think you can imagine a parenthetical element here:
The drawing-room began to look empty: the baccarat was discontinued
for lack of a banker; more than one person said goodnight of his own
accord (and [not only that] was suffered to depart without
expostulation); and in the meanwhile Mr. Morris redoubled in agreeable
attentions to those who stayed behind.
If that doesn’t ease your mind, here’s The Chicago Manual of Style again, in a Style Q&A that speaks directly to your predicate situation:
The comma isn’t necessary, but if you want to indicate a pause, add it
anyway . . . “Effective use of the comma involves good judgment, with the
goal being ease of reading.”
Lastly, if you have “an exquisite literary talent” (as Henry James once wrote about Stevenson), you can deploy a comma for any reason that suits your fancy.