Using Google Books, I found several instances of multi-task in print prior to 1966. Here's the earliest I found, in a description of a lawn mower from a 1954 issue of Chambers's Journal:
Last month a multi-task appliance for turf maintenance was described here, but it was pointed out that its size and cost made it more suitable for use on large areas of grass—e.g. sports grounds, parkland, etc.
I believe this shows that the term multi-tasking did not orginate specifically as computer jargon, but was present prior as engineering jargon.
This earlier use probably did not carry the meaning of performing multiple tasks at the same time, but rather meant, simply, able to perform multiple tasks.
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 no b 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:
crumb - O.E. cruma "crumb, fragment," from a W.Gmc. root of obscure origin (cf. M.Du. crume, Du. kruim, Ger. krume).
crumble - late 15c., kremelen, from O.E. _*crymelan_, presumed frequentative of gecrymman "to break into crumbs," from cruma (see crumb).
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 words thumb, numb, and dumb have unetymological mb in spelling, though they do not end in l. Why do they? In principle, the story begins in late Middle English. At that time, consonantal groups were often simplified. Solemn and column are now pronounced without n. In the speech of some people kiln is homophonous with kill, g is always mute in diaphragm, phlegm, and so forth. Because of the simplification, lamb was fleeced of its historical final b; today b is retained in spelling but not in pronunciation. The same happened to jamb, plumb, and tomb (borrowed words), along with womb (Old Engl. wamba “belly”; compare Wamba, the name of Cedric’s “fool” in Ivanhoe) and climb (Old Engl. climban).
As long as the simplification of consonantal groups remained an active force, literate people felt uncertain when to write m, as opposed to mb, and began to add b to m gratuitously, a mistake (here, reverse spelling) called hypercorrection. This accounts for the modern forms limb and crumb.
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.
Best Answer
Old English
In Old English, a language from the West Germanic family, the standard spelling was tunge (wiktionary) and the corresponding pronunciation was /ˈtʊnɡe/ "tun-ghe" (/ʊ/ as in foot). In other words, the final "e" was not mute but clearly pronounced. For instance in present-day German, you would say "Die Zunge" /ˈtsʊŋə/ (tsung-e). In both cases the final unstressed e is actually already a reduction of an older final a still present in many other cognates.
Middle English
The spelling of Middle English is strongly influenced by that of Old French in which the un sequence is uncommon. It was thenceforth often replaced by on which led to such spellings as tonge or tounge ('ou' being the French spelling of /u/ and /ʊ/) . Which can be compared to contemporary alternative spellings longe and lounge for the word long.
Modern English
With the evolution of the pronunciation of long vowels in Modern English, these spellings became highly misleading. "Tounge" for instance could be read /taʊndʒ/ (as in lounge). As a result, such spellings as tonghe or tongue were preferred. The fact that the simpler spelling tong was not preferred (in spite of the existence of such words as long) is probably a sign that the final e was not completely muted yet.
Finally, the tongue variant probably overtook the tonghe option as a result of an analogy with such words as L. lingua or languages.
Dutch
Also note that on the other side of the English Channel, the Dutch word followed a very similar path. From Old Dutch tunga it successively evolved as Middle Dutch tunge, tonghe and present-day Dutch tong.
Reference
This is the explanation offered by the OED regarding the spelling and the etymology of the word tongue.