You are being confused because of the misleadingly nonstandard pronunciation symbols being inexcusably thrust upon you by Merriam-Webster. It it in your best interest to ignore utterly any source whose pronunciation fails to use standard IPA.
Turning to a dictionary that does employ standard notation, the related word cactaceous, meaning per the OED:
Belonging to the old genus Cactus; or to the natural order Cactaceæ.
is normally pronounced /kækˈteɪʃəs/ according to that same source.
That tells you how Cactaceae starts out, but leaves you a bit in the dark about how it finishes up. Most of the remaining mystery lies in how the ‑aceae suffix can be pronounced several ways, depending on one’s formative education and personal predilection.
So for example, although I myself — and I am hardly alone in this — pronounce the word Cactaceae as /kækˈteɪʃiɑɪ/, other people are more apt to say /kækˈteɪʃieɪ/ or /kækˈteɪʃeɪ/ there instead.
I have never heard /kækˈteɪʃi/ but can imagine it existing. Probably very few still pronounce it as if it were Classical Latin, so /kɑkˈtɑkiɑɪ/ or even with some small effort to preserve the diphthong, /kɑkˈtɑkiɑe/.
Mostly what is happening is that the /si/ becomes (as it so often does) /ʃi/ there; think of spacious and many other such words.
I recommend to you the Wikipedia article on the ‘traditional’ English pronunciation of Latin. This is still true in general, but studious specialists of a younger generation sometimes do more in the way of maintaining the Latin diphthongs than our elders were wont to do.
Edit
As John Lawler astutely notes in comments, it may be important to preserve an unpalatalized /s/ instead of using /ʃ/ for the ‑aceae suffix in some contexts, because that suffix is a clear marker to botanists that one is referring to a plant taxon at the Family level.
That would make it end in /esie/ phonemically — or, with explicit phonetic diphthongs and glides as always occurs in English, in [eɪsijeɪ]. It really depends on how fancy you are getting with your notation, because those are the same thing.
The Anglophone botanists I know indeed use a simple /e/ (phonetic [eɪ]) for ae. I may hear the [ɑɪ] variant mainly from people who have spent more time with school-Latin than with actual English-speaking botanists. :)
"viva" meaning "long live" is borrowed from Italian or Spanish, and approximates the pronunciation of the source languages. (IIRC, the Vauxhall Viva car had the same pronunciation, though I don't have a source for that.)
However, "viva", short for "viva voce", meaning "oral examination", is borrowed from Mediaeval Latin. Pronunciation rules for Mediaeval Latin words are not the same as those for modern Italian and Spanish, though I don't know when or why the long 'i' prevaled for this sense.
Source: http://dictionary.reference.com/browse/viva
Best Answer
All of the words you provide (limb, thumb, crumb) are listed in my local dictionary without a
b
sound. Things seem basically the same with suffixes (i.e. thumbed has nob
sound).Crumbled is is a completely different word and receives the pronunciation typical for "mble": thimble, tremble, fumble, etc.
It is true that crumb and crumble have common origins:
The "b" in each word appeared later (mid-15c for crumb and 16c for crumble). Given the typical pronunciation of the "mb" and "mble" it isn't surprising that these words are treated the same. Why they received a "b" at all is a little vague from the notes in the above link.
Digging deeper I found Anatoly Liberman who gave a more thorough explanation:
The rest of that article is extremely fascinating and highly recommended. Also relevant are words like limber, timber, combine which most certainly pronounce the
b
despite the "mb". So are other cases of a silent "b": debt, subtle, etc.