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.

Mervyn Peake's use of ‘must have’

I've just finished Titus Alone, the black sheep of the Gormenghast trilogy.  Peake's command of the language is impressive, despite the erratic punctuation, so I was especially intrigued by his peculiar use of the modal must in phase-modified clauses, that is, his use of ‘must have V-en’.  This form of words usually encodes an inference as to fact, as in Withnail's ‘What happened to my agent?  Bastard must have died.’  But now look at these (pagination from 1998 Vintage editions):

Titus Groan
‘The sky was overcast and had he not made this same journey through the darkness a thousand times he must surely have lost himself in the night.’  (The Library, p. 206)

‘Her fever had raged, and but for the care with which the old man watched over her she must surely have died.’  (Farewell, p. 357)

‘Had the flesh, the fibres, and the bones of the chef and those of Mr Flay been conjured away and away down that dark corridor leaving only their four eyes suspended in mid-air outside the Earl's door, then, surely, they must have reddened to the hue of Mars, reddened and smouldered, and at last broken into flame, so intense was their hatred – broken into flame and circled about one another in ever-narrowing gyres and in swifter and yet swifter flight until, merged into one sizzling globe of ire they must surely have fled, the four in one, leaving a trail of blood behind them in the cold grey air of the corridor, until, screaming as they fly beneath innumerable arches and down the endless passageways of Gormenghast, they found their eyeless bodies once again, and re-entrenched themselves in startled sockets.’  (Early One Morning, p. 365)

Gormenghast
‘Had he thought himself awake he must surely have pursued, however faint his hope of overtaking the slender creature.’  (§19, p. 127)

‘Had his exile in the woods not inured him to loneliness, then he must surely have found these long days insupportable.’  (§45, p. 292)

‘For Titus to have seen a tenth of it must have taken the edge, not off his wonder or speculation, but off the shock of pleasure that he was finally to receive when evening came.’  (§50.i, pp. 312f.)

‘Had she as a girl been naturally joyous yet all that had befallen her must surely have driven away the bright birds, one by one, from her breast.’  (§75, p. 445)

Titus Alone
‘This gun-boom had come just in time, for had it been delayed a moment longer Titus must surely have been grabbed and questioned.’  (§20, p. 35)

‘Some, in his place, must surely have seen battle or the great jaws of carnivores or landscapes of infinite mystery and invention complete with bridges and deep chasms, forests and craters.’  (§49, p. 113)

‘Had it not been that he took up the rearguard station his facial fatuities must surely have maddened his two companions.’  (§78, p. 180)

This list may not be comprehensive, but it'll do.  In each case I find that must jars, and my ear cries out for would instead.  I don't think that this is merely a matter of stylistic preference, but I can't be sure, so I would be interested to hear of similar examples from elsewhere.