I think "right" or "wrong" are a tad strong for most of the cases 
sited. But as a professional book designer and typographer, there's 
unquestionably "better" and "worse."

For improved legibility, inter-sentence space should generally be a bit 
greater than inter-word space.

Typewriters only had one distance they could travel. Either 1/10th of 
an inch ("Pica") or 1/12th ("Elite"). So the only way to add extra 
space after a sentence was to double it. That's way too much extra 
space, but it was generally better than the alternative. The real 
problem was that the words were too far apart, not that the sentences 
were too close, but again, the fixed spacing was already an abominable 
situation.

Proportional type, dating all the way back to Gutenberg, would 
generally use 1/3rd or 1/4th of the height of type type as the 
inter-word spacing. This would usually work out to about the width of a 
lower case "t" or "l".

When setting modern (by which you may also read "all type before 
typewriters" as well) proportional type in fully justified form (left 
and right margins both even), the spaces must be stretched out on a 
line-by-line basis to fit. Really good typesetting programs (and really 
good typesetters sticking little bits of lead between their words (and 
I've done that, too)) will add more of the space between sentences than 
between words, so as the line stretches, the inter-word space to 
inter-sentence space ratio actually changes. (Take a look at a narrow 
newspaper column sometime.)

More sophisticated approaches to space will ignore a user's attempt to 
sprinkle extraneous space in. Less sophisticated ones might allow it, 
and even treat them as individual spaces, stretching both of them 
during expansion. {shudder}

The fact that both the MLA Guidelines and the Bedford Handbook 
encourage poor typography is regrettable. ("If you cannot type 
appropriate punctuation, e.g. an em-dash or en-dash, please use 
appropriate substitutions. For both dashes, substitute a pair of 
hyphens, which, like true dashes, are typed without adjacent spaces." 
There's still software out there that will happily wrap a line between 
the two hyphens. Ick!) Nevertheless, if you're submitting a paper to an 
institution that expects or requires that, then to not follow them is 
wrong, even if the legibility of the submission is better.

What it all boils down to is "Putting two spaces after a period at the 
end of a sentence is an artifact left over from the days when the 
typewriter was the prevalent text-making tool. Unless you have a 
specific reason or requirement to do otherwise, it's preferable to put 
only one space between sentences."

*****

For breaking text into sentences, sometimes I find it easier to work 
backwards.  Also, only very colloquial writing will have  a one-word 
sentence, so you can solve all "Mr./Dr./Ph.D." cases by the fact that 
if a word starts with a cap and ends with a period, it's not a 
sentence. For a more sophisticated approach that's still not too 
complex to program, check the final word of a sentence against a 
dictionary. If it's found there without a final dot, then you're almost 
certainly looking at the end of a sentence. If it isn't, then is it 
found anywhere else in the document without a dot? If not, then you're 
probably looking at an abbreviation. (My mail program uses a monospaced 
font. If I thought most readers would read it with a proportional font, 
I'd have typed "Ph. D." above, since it should have a thin space before 
the D.)