Must have led a sheltered life

I have previously queried Peake's and Coleridge's use of ‘must have V-en’ in the main clause of past-tensed conditionals, e.g. ‘the Ledge at the bottom was [so] exceedingly narrow, that if I dropt down upon it I must of necessity have fallen backwards & of course killed myself.’

After a quick search for ‘must surely have’ in the British National Corpus, I'm reluctantly coming round to the idea that I have indeed just led a sheltered life when it comes to this construction.  Witness:

‘But for the Munitions of War Act of July 1915 which enabled the Board of Trade if necessary to impose arbitrated settlements on unwilling employers, the union's policy of patriotic co-operation must surely have failed’ – Arthur Marsh & Victoria Ryan, The Seamen: A History of the National Union of Seamen (1989).

‘Science and technology must surely have progressed in a different way if these principles had been embraced from the start’ – Storm Constantine, Hermetech (1991).

‘Had it not been for their masks, the Phantasms' faces must surely have blistered – a gulf of rising furnace-air yawned beyond that hatch’ – Ian Watson, Warhammer 40,000: Space Marine (1993).

‘Had it not been for the psychic tracer, they must surely have lost themselves in the labyrinthine entrails of what was not one vessel but many’ – Ian Watson, Warhammer 40,000: Inquisitor (1993).

I don't know whether to make anything of the fact that three of these are from the trashier end of sci-fi (and that two are even from the same author) even though the search was conducted over a wide variety of sources.  If I ever find the time, I'll try wading through the vastly more extensive search results for ‘must have’ simpliciter.

Samuel Taylor Coleridge's use of ‘must have’

In an earlier post I considered Mervyn Peake's use of the modal must in phase-modified clauses.  The ‘must have V-en’ construction usually encodes an inference as to fact (‘What happened to my agent?  Bastard must have died’), but Peake also uses it where I'd expect the modal would, e.g. ‘had he not made this same journey through the darkness a thousand times he must surely have lost himself in the night.’

Well, I've just stumbled across the same phenomenon in Coleridge's description of his death-defying descent of Broad Stand in 1802:

‘the Ledge at the bottom was [so] exceedingly narrow, that if I dropt down upon it I must of necessity have fallen backwards & of course killed myself.’

A rummage turns up another example from ch. 14 of his Biographia Literaria (1817), discussing the turn-of-the-century Lyrical Ballads:

‘Had Mr Wordsworth's poems been the silly, the childish things, which they were for a long time described as being; had they been really distinguished from the compositions of other poets merely by meanness of language and inanity of thought; had they indeed contained nothing more than what is found in the parodies and pretended imitations of them; they must have sunk at once, a dead weight, into the slough of oblivion, and have dragged the preface along with them.’

And finally, again from the Biographia, in Satyrane's Letters no. 1:

‘And after dinner, when he was again flushed with wine, every quarter of an hour or perhaps oftener he would shout out to the Swede, “Ho!  Nobility, go—do such a thing!  Mr Nobility!—tell the gentlemen such a story,” and so forth, with an insolence which must have excited disgust and detestation, if his vulgar rants on the sacred rights of equality, joined to his wild havoc of general grammar no less than of the English language, had not rendered it so irresistibly laughable.’

Maybe I've just led a sheltered life when it comes to this construction.