were late / would be late

Oh my, I'm so surprised! I would never have thought you said it yourself! By the way, you said "If I'd have known", and not "If I'd known" (which can't mean "If I would have known", because you would be leaving out a syllable...).
Anyway, I'll tell you the truth: I've always thought it was non-standard, same register as "ain't". Low register, only common in certain dialects. I don't remember a single grammar book saying it was acceptable. In fact, I only remember people criticizing it. Here are a few comments like the ones I've always heard:

Also in the midwest, I often heard, "If I would have . . . ., I would have . . . . ."  Drove me nutz! (Avangi)
http://www.englishforums.com/English/CondtitionalTense/zndwv/post.htm#482524

The construction, actually any construction with 'would' in a hypothetical if-clause, is considered non-standard. (Jim)
http://www.englishforums.com/English/HadHaveHad/chrcq/post.htm#201527

To me, it evokes comical people from remote mountain regions or from city slums who are uneducated and have some accent that most listeners dislike. But maybe research would reveal it to enjoy a wider demographic distribution. To my ear, it's about as obtrusively wrong and amusing as 'them' for 'those'. It's definitely not acceptable for standard written usage. On the other hand, it's not the most awful mistake one can make. (Native from San Diego)
http://forum.wordreference.com/showthread.php?t=97201#3

Would have for had - In spoken English, there is a growing tendency to use would have in place of the subjunctive had in contrary-to-fact clauses, such as If she would have (instead of if she had) only listened to me, this would never have happened. But this usage is still widely considered an error in writing. Only 14 percent of the Usage Panel accepts it in the previously cited sentence, and a similar amount—but 16 percent—accepts it in the sentence I wish you would have told me about this sooner. (A Practical and Authoritative Guide to Contemporary English)
http://www.bartleby.com/64/C001/061.html

On the other hand, some people seem to use it all the time, regardless of region or cultural influences, and so it's a common feature of informal English to them. One example:

Hmmm...never thought of that construction as a mountain dialect but normal everyday American! I never knew it was incorrect! (Native from the Midwest)
http://forum.wordreference.com/showthread.php?t=97201#5


That's why I was so surprised, considering you are a writer and knowing you use "correct" English most of the time. This can only mean two things now... I have to choose between:
1) I start to use it as well, feeling ashamed of not knowing is was so common.
2) I don't start to use it, and I start to tease you because it's not like you to speak bad English.

LOL, just kidding. Seriously, I don't know what to say. I think I'll consider the fact that people are actually more tolerant of it than I thought, so it's not that bad. But do you also use it in your writing? You know, just because I don't like prescriptive grammar, doesn't mean I don't want to know about it. In fact, prescriptive grammar is part of descriptive grammar.
I'll appreciate any opinions on this. Thanks.

Wow, cool. I would never have thought it was ok in writing... Thanks.