Learn English – Differences between “stupid to the last drop” and “stupid”

From New Oxford American:

dark horse |ˈdɑrk ˈˈhɔrs|
noun
1 a person about whom little is known, esp. someone whose abilities and potential for success are concealed : [as adj.] a dark-horse candidate.
2 a competitor or candidate who has little chance of winning, or who wins against expectations : a preseason dark horse as the nation's top collegiate football team.

sleeper |ˈslipər|
noun
3 a movie, book, play, etc., that achieves sudden unexpected success after initially attracting little attention, typically one that proves popular without much promotion or expenditure.
• an antique whose true value goes unrecognized for some time.

From Etymonline:

dark horse
in politics, 1842, an image from horse racing, in which dark is used in its figurative sense of "unknown."

Moonraker is called a "dark horse"; that is neither his sire nor dam is known. ["Pierce Egan's Book of Sports," London, 1832]

sleeper
[...] Sense of "something whose importance proves to be greater than expected" first attested 1892, originally in Amer.Eng,. sports jargon, probably from earlier gambling slang (1856) sense of "unexpected winning card."

Here's some generalizations gleaned from the above:

• dark horses are usually people; sleepers are usually things.
• dark horses are intentionally kept unknown; sleepers are simply not promoted.
• a dark horse might achieve success; a sleeper already has—surprisingly.
• dark horses' limelight often decrescendoes; sleepers' crescendoes.

As to your questions, I'd say they're not quite interchangeable. Knowing their different connotations will likely prove one to be the better word choice.

A dark horse might also be called an enigma, or simply an unknown; while surprise hit, cult classic, and box-office success are other phrases often applied to sleepers.

I presume you know the definitions of former, only, and last.

They all mean what they usually mean. However, when used in series like this, it serves as a means of intensification. It does clarify that she has no intention or ever remarrying, but it's emphatic. It is a variation on the more common first, last, and only series.

Someone who hears "He is my only child" will know that the child was also the first and last child. However, "He is my first, last, and only child" makes it crystal clear that the speaker feels strongly about something (without context, I'd guess her child was enough for one lifetime.)

Similarly, my former and only husband clearly means he was her last husband; only and last means former husband as well, but the three in a series indicates something about the mood of the speaker. Clearly she means to say something not only about her husband, but also about the institution of marriage. If not, she could simply have referred to him as her ex-husband.

examples of this use:

• First, last, and only structural selectors provides you with in-depth training on Web.
• The First, Last and Only All-Black Ranger Unit
• The First, Last and Only Thing You Need to Hear
• Mandalay Bay is the first, last and only resort.
• Live Each Day as if it were Your First, Last, and Only Day
• The first, last, and only choice for the Senate...