Certainly the origin of the more common contemporary US sense and use of the phrase 'dead man walking' must be understood in the context of all forms, senses and uses of the phrase, current and historical.
Among the forms of the phrase, I include these:
- dedman/dedeman/deadman/dead man walk/walks/walked/walking
- inverted and plural forms of the previous (for example, 'walking dead men')
- Deadman's Walk
Many of these forms are scarce or nonexistent in the textual evidence readily available for examination, and so were relatively easily assessed with an eye to their influence on the development and use of the common US phrase.
The spelling variants 'dedman' and 'dedeman' were suggested by early attestations (a 1400, c 1440) given with the entry for "deadman, n." in OED:
(deadman). = dead man: formerly written and pronounced as one word. (Cf. blindman n.) Obs. exc. in names, as Deadman's Walk.
Fortunately for what little remains of my sanity, forms of the phrase using the early spellings ('dedman', 'dedeman') were nonexistent in full-view works in the HathiTrust Digital Library corpus.
Three senses and use-contexts of the phrase, current and historical, are especially pertinent in light of the question asked. Those have already been mentioned in the question, but for clarity I'll reiterate them, slightly reworked and elaborated, here:
- Religious senses and contexts. It was suggested that religious writings might be the origin of the phrase.
- In the US and South Africa, and especially in the context of use in US prisons, the phrase is reputed to be or have been used to announce or otherwise refer to "a condemned man walking from his prison cell to a place of execution" (Farlex Dictionary of Idioms, 2015, and Collins English Dictionary, 2017).
- In general, common and informal contexts, by "extension", the phrase is used to refer to "any man who is in great trouble or difficulty and is certain to face punishment, especially the loss of a job" (op. cit.).
The form "Deadman's Walk" could be dispensed with out of hand, were it not for the following historical account concerning Oxford and environs, published in the Oxford Journal, 13 February 1886 (paywalled):
"Over against Logic-lane," says Wood, "there was a way through Alban Hall Walkes, where yet in the towne wall is the signe of a doore to goe into the meade." Here and there are still to be seen the crowns of the arches upon which the wall hereabouts was built, and on the south side of the wall is that sheltered retreat from which it is said to have derived its name of Deadman's Walk. I was informed, however, by the late General Rigaud that the walk acquired that name in consequence of the execution of Col. Windebank, who was shot under the Merton wall for surrendering Blechingdon House to the Parliament forces. On the other hand the late Mr. Davenport says Col. Windebank was shot within the precincts of the Castle. I have searched the pages of Rushworth as the most probable author in an attempt to clear this discrepancy, but in vain, and I must leave the task to others.
As is apparent from the account of General Rigaud, supposing it to be true, Col. Windebank could be said to be the prototype 'dead man walking' to his place of execution, an event which is memorialized in the name of the walkway itself. Whether or not this event or the name of the walk in any way influenced the later development of context-sense 2 (above) in the US and South Africa, the name of the walk at least demonstrates that the essential semantic and collocative elements of the phrase, that is, essential elements of the phrase's origin, were already in use when Deadman's Walk was named, presumably sometime in the late 1600s or early 1700s.
Use of forms of the phrase in religious contexts appears at least as early as 1654. Such uses as precursors of context-sense 2, the contemporary US-South African sense, seem to offer little more than demonstration that the collocation has been in use for centuries. As precursors of a generalized context-sense 3, 'a man in great trouble or difficulty certain to face punishment', the lineage seems more exact and pertinent, as will be shown in the examples of forms of the phrase in religious contexts.
It should be noted at this juncture, however, that context-sense 2, 'a man facing execution' is itself a highly specific and distilled version of the generalized context-sense 3, 'a man in great trouble or difficulty certain to face punishment'. In consequence, I would expect to find examples of the general sense (3) in use, and applied in a variety of contexts, before the development of context-sense 2, and this was the case. Among those examples are early uses of the phrase in religious contexts.
- Where you say that [without faith a man can no more Receive Christ, nor do ought towards it, then a dead man can walk or speak.]....
- That [without faith, such can no more do ought towards the receiving of Christ, then a dead man can walk or speak]....
Rich. Baxters apology against the modest exceptions of Mr T. Blake, 1654.
In the example given, a clear figurative parallel between the troublesome plight of a man without faith and a dead man walking (or speaking) is drawn. Again, the essential semantic and collocative elements of the phrase 'dead man walking' (context-senses 2 and 3) have been deployed. So also in the next example from a religious context, although here connection between the troubled melancholic and certain punishment is not so clearly drawn:
The Sanguine man tels the Melancholick of faction, separation and pride, and sayes he is like the Pharisee, that justified himself and despised others. He tels him, his name is Stand farther off, which saith, because he thinks that he is holier then others. He accuseth him of malice and spitefulness, and faith, that Love is much decayed, since these pure Gospellers came up. He hates him as the Enemy of Mirth, yea the enemy of Life, for he holds him a walking dead-man, and doubts he seeks to make others as dead as himself.
Treatises and meditations, dedicated to the saints, and to the excellent throughout the three nations, Francis Rous, 1657 (emphasis mine).
Moving from the grandeurs of religion and capital punishment of the soul or its corporeal vessel to the trivia of personal finance, the evidence shows aphoristic use of the phrase 'dead man walking' before the development of context-sense 2, rather than after it. After was indicated by the Collins reference to "by extension" for the "general, common and informal" sense "any man who is in great trouble or difficulty and is certain to face punishment, especially the loss of a job" (emphasis added), yet these uses were earlier:
All the rest of the world went mad; but I went on quietly, creating confidence and credit — a man's best fortune in life, which, once lost, can never be recovered, and which, in business life, is better than goodness — yea, even than life itself; for it is the breath of a business man's nostrils, and without it he is but like a dead man walking.
John Horsleydown; or, The confessions of a thief, Thomas Littleton Holt and Hablot Knight Browne, 1860 (emphasis mine).
"Perdio," added his companion, "a man with money is a man in the right. So put that in your pipe, amico mio, and smoke it. Ay, money, it's like one's other blood; a man with empty pocket, 't is but a dead man walking."
Vestigia, George Fleming, 1884 (bold emphasis mine).
Finally, consider this 1919 account from a man with a view from inside:
The true House of the Dead is an English jail. There to give a crust of bread to a fellow or pass a word of cheer are crimes. Prisons, as I have said before, are not houses, they are sepulchres. Dead men walk in them, with expressionless masks of death on their faces, and silence, the brother of death, on their lips.
A second chronicle of jails, Darrell Figgis, 1919 (emphasis mine).
It's true, of course; prisons are a form of death in life, and prisoners are dead men walking, whether or not they are on their way to execution.