Your points are well-made except that sometimes, for clarity, I think it’s necessary to use an apostrophe to pluralize. I think it’s clearer to the reader if it’s “Oakland A’s” rather than “Oakland As”.
Yes, the apostrophe is allowed in that situation. I use this conversation as an example: "What did you get on your report card?" "A's and B's." But if the letters were B and C, you could omit the apostrophes!
A consideration re. your Oakland A's example is that the team does use the apostrophe. So the apostrophe is not only correctly used for written clarity, but it also conforms to the accepted usage for that particular proper noun.
The other situation where an apostrophe is correctly used to make a plural is when you're referring to plural numbers of particular words. "He used six and's, four but's, and two so's in that nightmarishly long sentence." The apostrophes are needed because he used the word "and" six times, not the word "ands."
If you look at different grammar and publishing texts, you'll see that different sources disagree on many points of usage. And usage changes over time. I'm old enough to remember when apostrophes were used to make plurals of acronyms: PC's, for example. Now we're supposed to write PCs.
In the really olden days when libraries had card catalogs, there were elaborate rules as to how acronyms were alphabetized and filed. That is now so far in the past that I can only remember that acronyms were separated into two categories: those which were pronounced as words (UNICEF, NASA, NATO, AIDS), and those which were pronounced as letters (USA, GPS, HTML, NAACP) -- and the two categories were filed differently.