I'm team single space and we seem to be the PC generation as the type writer mandated the two spaces.
I matriculated in a creative writing program which, anyone familiar knows, is full of poor grammar and an over-reliance on editors to whip punctuation/grammar into shape without losing the punch of the ideas and stories created. In that world the semicolon is taboo and viewed as amateur, unless shooting for camp or irony/satire, but I agree with you in that it isn't used enough. Alternatively, the em dash is used and abused (and I'm one of the worst offenders) to the point where writers meme'ing about the em dash has gone mainstream.
I matriculated in a graduate business program which, anyone familiar knows, pays lip-service to APA style guidelines. The citations are either given to you in advance via case studies and academic articles/business cases or you just watch a video every other day to use citation monkey because nobody really cares if your treatments are stylistically correct due to the lack of original research and lack of emphasis or care to publish.
I agree emphatically with your 4th point and would only add that effective communication can also vary by culture and by industry, e.g. Young professionals can say a thousand words with memes, gifs and emoji's, anyone who has seen /bpt or facebook understands there are certain spellings and casual sentence structures that are common and normalized in inner cities and/or certain demographics, and anyone on Wall Street or PE or Management Consulting understands that an email or text from an MD or Partner that says the dreaded "Pls fix thx" means you are in a world of crap (again, to the point of being a meme).
All that said, it blows my mind that anyone would take issue with using a period to form a declarative sentence, especially for front-office/customer facing folks who need to exhibit executive presence and polish. I can see back-office and internal communication being sloppy, but I don't understand, to the point of being skeptical and essentially not buying this premise, that the period is some weird thing now.