I think OP has the issue of "appropriateness" the wrong way round.
It's quite common in Britain not to address doctors and professors by that title. They're just plain "Mister", the same as the rest of us. I can't say exactly why - maybe we're a bit more egalitarian.
Clearly the change was made specifically for the US market, where I guess this usage is considered somehow "impolite" (disrespectful to either the office or the holder of the title).
I must admit I find it odd that such a change should be made to an English-language book in this way. But obviously the UK wording came first, so arguably the onus is on Americans to explain why they worry about such niceties (or why their book publishers think they might worry).
Wikipedia has what seems to me to be a very complete write-up on the pronunciation of words like "tutor," "news," "brew," that are not pronounced with a "y" sound for many speakers, even though the spelling and history would suggest that pronunciation. This phenomenon is named "yod-dropping," as "yod" is one name for the "y" sound. In general, it is possible to predict which words are affected, although the rules are fairly complex and there are some areas where there is variation even among speakers of the same regional variety.
For this reason, I won't try to list them all here and risk giving a simplified, wrong picture–you can find them over at Wikipedia, or if you look at phonologists' work on describing "yod-dropping."
So to answer your first question: the pronunciation with "y" is the original one, and has the same origins as it does in words like "feud" or "fume" where both British and American English speakers standardly have a "yoo" sound. The pronunciation without "y" is newer, but I've had a hard time finding actual dates given that correspond to the timeline of this change. It appears that yod-dropping occurred earlier or later depending on the particular phonetic environment, and the change is still in-progress.
Regarding the second part of your question: the consonant "y" is pronounced towards the front of the mouth. For many American-English speakers, there is a tendency for the ordinary "oo" sound to be pronounced more towards the front of the mouth after coronal consonants, a class that includes /t, d, n, s, z, l/ among others (Source: The Atlas of North American English, Sound Changes in Progress, the fronting of /uw/ after coronals). It seems possible to me that this tendency first led to confusion between "yoo" and "oo" after these sounds (for example, in the pair of words "do" and "dew"), and then led to what phonologists call neutralization: a complete lack of contrast.
Best Answer
When the English settlers landed in the New World, they didn't have a word for maize. Maize is a New World crop which was unknown in Europe. The word "maize" was originally Spanish, and comes from the word "mahiz" in the Arawak language of Haiti, and in the early 1600s it was not yet a common word in England. The settlers called it "Indian corn", which soon got shortened to just "corn".
EDIT: In the comments, some people are questioning whether "Indian corn" and "maize" refer to the same thing. They certainly don't today; in the U.S. Indian corn usually means ears of maize with multicolored kernels which are grown primarily for decoration. However, both terms were used and appear to have been treated as synonyms in the U.S. during and before the 18th century. From a section of An Universal History (London, 1763) discussing New England: