Speculatores

[M. Caecilius]

Alternatively, might some of them have been supernumery centurions on the legion's books, who rather than commanding their own centuries functioned as part of the headquarters staff as independent intelligence officers?

They could be, but I wonder whether they would be "centurio speculatorum" in that case, rather than "centurio praepositus speculatorum" (or similarly defined as a special duty), like those who were given numeri, vexillationes, or even exploratorum, attested in some inscriptions - unless we have to see here a career move, and they were centurions before being upgraded to praepositus. That's pretty much speculation about Roman procedure, though, so I won't argue this very loudly!

Also, if we assume that there were ten or so speculators on the strength of the legion, these men would presumably still have been mustered on the rolls of their centuries, like the legion cavalry, so a speculator saying that he was a member of a certain century doesn't have to mean that the whole century was composed of speculators!

That seems likely and eliminates AE 1926,4 as evidence for independent units, but as you say yourself, the centuriones speculatorum Augusti are attested in inscriptions. As you intimate above though, centurions don't necessarily mean there were full centuries underneath them, rather than a few men taken from the rolls, and they often found themselves posted throughout a province on special duties, assisted by a few men, but I do wonder whether the combined evidence cannot be used to make a case. A shaky case, sure, as your objections show, but a case nevertheless.