Q: In 2006, Andrew Strauss was famously referred to (by himself) as "the stand-in's stand-in". Could there truthfully be said to be any other occasion where a man appointed specifically as a replacement captain then fell unfit himself and someone else took his place?
A :Really, there are only two cases where this could be said to have happened. In 1948, England toured West Indies; in those days it was still far from unheard of for the captain, who was invariably an amateur, to be unable to make certain tours due to business commitments. And England's captain Norman Yardley was not able to tour that spring; the 45-year-old Gubby Allen was picked instead as the stand-in, but he missed the First Test with illness. Ken Cranston was the man picked as the third-choice that time. MacLaren, as mentioned, missed a couple of Ashes due to his own business commitments; one such was 1907/08, and the man picked as tour captain in his place, Arthur Jones, missed the first three Tests, where Fred Fane took the position.
Q: What is the longest (in terms of matches) England captaincy which did not require a stand-in?
A: The same as the longest spell of all. Remarkably, Michael Atherton's 52 Tests in charge were completely uninterrupted - more remarkably still, perhaps, given that Atherton suffered almost all career from a debilitating, degenerative condition, Ankylosing Spondylitis. This forced him out for several spells either side of his captaincy stint.
Q: Apart from David Gower, could anyone be said to have had the England captaincy given to them for two separate stints (on the basis of Mike Brearley's return midway through 1981 being something everyone knew really was only a stopgap arrangement)?
A: Really, only the aforementioned Colin Cowdrey. Cowdrey inherited the leadership very briefly after the summer of 1966 had already begun, before Brian Close was preferred (on an almost equally brief basis) later on. Cowdrey then took the captaincy back from Close and had a slightly longer stint, in the spring and summer of 1968 and winter of 1968/69. A few others have come back to captain as stopgaps after their official tenure had been terminated (Arthur Carr, Allen in the aforementioned case, and more recently Alec Stewart and Atherton).
Q: Are 1988 and 1966 unique in being summers where more than one England captain was replaced?
A: Depending on what "replaced" means, yes and no. In 2001, England were led by three men as they had been in 1966 - but both Stewart and Atherton were standing-in for the injured Nasser Hussain, and the "replacement" was on the basis of "until Hussain returns". In 1988 and 1966, the deliberate decision to change the captaincy (rather than forced change through absence of the first-choice man) was made three times and twice respectively - infamously, Mike Gatting was sacked after a single game before John Emburey, Chris Cowdrey and Graham Gooch led briefly (Derek Pringle also captained as an in-match replacement before Gower was reinstated in 1989). In 1966, MJK Smith too was removed after a single game, Cowdrey led in the middle three then Close was picked for the last one (and the entire following summer, before Cowdrey returned in the Caribbean). In no other summer has this happened more than once.
Q: Since the 1930s, how many people have captained England as stand-ins without ever being handed the armband in full?
A: That decade started with the last of an instance which had been relatively commonplace; Harold Gilligan and FSG Calthorpe captained two sides, neither of which were regarded as England Test teams at the time, in West Indies and New Zealand respectively, and the matches were retrospectively made Tests. England's proper captain at the time was APF (Percy) Chapman, and Bob Wyatt stood-in for him at the end of the summer of 1930. Wyatt would go on to lead England; Cyril Walters, who stood-in for him Wyatt 1934, would not. The aforementioned Cranston comes next, and George Mann too led in South Africa in 1948/49 when Yardley was again unavailable. Donald Carr deputised for Nigel Howard in 1951/52; David Sheppard for Hutton in 1954; Tom Graveney for Cowdrey in 1968; Tony Lewis for Raymond Illingworth in 1972/73; John Edrich for Mike Denness in 1974/75 (this was unique in being a case of the touring captain dropping himself); Geoff Boycott for Brearley in 1977/78; Allan Lamb for Gooch in 1990 and 1990/91; Mark Butcher for Nasser Hussain in 1999; and Marcus Trescothick and Flintoff for Vaughan, in 2004, 2005/06, 2006 and 2006/07. So, with Walters, Cranston, Mann, Carr, Sheppard, Graveney, Lewis, Edrich, Boycott, Lamb, Butcher, Trescothick and Flintoff, that is 13.