Interesting review here of the Lost Legion book:
O'Reilly, Lost Legion Rediscovered
As for the legend itself, it's been pretty comprehensively critiqued by Denis Van Berchem and David Woods. You don't have to accept Wood's ingenious suggestion for the origin of the story (an attempt to influence the loyalties of the genuine Thebaei legion during the reign of Eugenius) for his overall criticism of the legend itself to be valid:
The Origin of the Cult of St Maurice
There are three rather essential problems with the legend as it stands:
1. The massacre was supposed to have happened around 285, but the supposed prohibition of Christian worship in the army dates from some ten years after this, and the penalty was dishonorable discharge, not death. The persecution edicts, compelling sacrifice, didn't appear until 303, and were not widely applied in the west anyway. Earlier military martyrs were killed for insubordination, not Christianity per se.
2. There are no recorded Egyptian troops in the west at this point. In fact, papyri from southern Egypt record large detachments of troops from the Danube operating against rebels in the Thebaid in 293 and 299 (nobody, as far as I know, has tried to connect these rebellions to the story of the Theban legion, btw!).
3. The killing of such a large body of men (especially as decimation had not been used in the Roman army for several centuries) could not have been 'covered up' by the state. (Conspiracy theorists and amateur historians are very fond of the idea of the state 'covering up' this or that!) The bagaudae revolt was not a minor matter of tax evasion but a serious threat to central government, and Maximian (who was appointed Caesar specifically to deal with it) would surely have needed all the troops he had, and not risked the demoralising effects of such savage punishment upon the rest of his force.
