The indiscriminate use in New Zealand of "signage" to mean sign or signpost means not that it must be fine because they do it, rather that the poor people of that benighted land have been inveigled, by their own example or maybe the fantasy that they are doing American English or computer-speak, into using the horrible examples you have quoted here. They do a lot a more violence to the English language than this: I have heard "farewell" used as a verb (Today we farewell Mr Smith), "exit" as a verb (please exit the hall) and "swari" as an evening party, which I must surmise is a Maori word, although I suspect it is really ignorant-speak for "soiree". Do not under any circumstances take their mangled version of the language as a model, nor is it even an amusing subject for laughter, as Australian is.

Have now done so. Cracking good read! I suggest that you use it as your "bible" for US English.

Merriam–Webster's Dictionary of English Usage is a usage dictionary published by Merriam-Webster, Inc., of Springfield, Massachusetts. It is currently available in a reprint edition (1994), I gather. It is the one which Bob & Bing once sang about, being, like them, Morocco-bound. I reckon it is as American as apple pie.
Now, when a writer uses forms of English other than Standard English, does it mean he doesn't know any better? Or is he ascribing to his characters a bit of a regional dialect, perhaps?
Should you be "done", as we say in Britain, for some heinous crime, would you be happy to be "sent down" for a lengthy prison sentence by a judge speaking, say, in the language of some of the characters from Huckleberry Finn, one of my favourite novels? Or Walter Scott? As long as he had a copy of Webster's among the other tomes lined up on his bench? In such unhappy circumstances I would prefer that the pronouncement be made by someone dressed in formal robes and speaking Standard English. If he were to fail to use the subjunctive where indicated I would lodge an objection, for sure.

'If I was a cad' allows the possibility that maybe I was indeed a cad. In that case, if 'I would apologise' is subjunctive, because conditional, it suggests that I would but I am not going to, because a further condition would first have to be satisfied. So:
"If I was a cad, I would apologise for it if you were to persuade me."

Goofy
English usage and correct (as you say, formal) English are not the same thing. I started my correspondence with this website suspecting that many misuses of correct English by the British come from North America as here mentioned by Perfect Pedant. My suspicions have not been allayed, confirmed rather!
It is accepted that there are regional dialects and a wonderful thing it is that this is so, of course. Conversations often feature clumsy English, naturally. But when public figures are making gross errors of standard English through carelessness at times when there is a reason to get it right, it is alarming. Are errors in printed headlines, or legal documents, or school textbooks, acceptable? Speeches by politicians? You get my drift.

To think to act, meaning thinking of acting, wondering whether to act ...

To think to act soon, or maybe to put it on hold.

Birmingham is indeed a beautiful city, when viewed with one's eyes shut, also with eyes open, viewing the place in the rear-view mirror, retreating into the distance.

Re: “If I was” vs. “If I were”
Perfect pedant:
If I was an ass I am sorry. If I were you I would give up. "If I was.." refers to fact, or possible fact. "If I were..." refers to impossible (closed) condition. I can't be you. But maybe I was an ass.
Timbo: you are right, surely no one else is reading this stuff by now.

Nan,

really enjoyed the one about getting one's drawers in a wad about it. In England it is "getting your knickers in a twist", by the way, but I like your phrase better.

Yes, I know all about teachers. Too much! They don't like it when their own poor command of the language is noted. I should know. They do not feel it is "trendy" or "enjoyable for the pupils" to worry about such trivia, so to "make it fun" they ignore it. So sad.

Keep the comments coming!

Othello: Iago: Desdemona:
"beware the green-eyed monster, jealousy!"

The "it" nicely summarises the first clause to act as a concise subject of "doesn't mean", otherwise if you leave it out (your second sentence) there is a long (seven word) clause to act as subject. "That" is not as good a choice as "it" in my view. Your first sentence ("it doesn't mean") is the best. To avoid it altogether, as suggested, by rearranging the whole thing, is an artifice, well, to avoid the problem! And "doesn't" is just fine, nothing wrong wrong with it. The whole thing is idiomatic and perfectly sound grammatical English.