Re En Garde!, it was discussed at length in What Is Dungeons & Dragons? by Butterfield, Parker and Honigmann (Penguin, 1982) and compared to D&D in terms of mechanics, differences in base assumptions (abstract levels versus social standing, for example) and style of play. The authors were clearly fans and certainly seemed to regard it as simply a different sort of RPG.
As far as the game’s authors go, as far as I know it was written by Frank Chadwick and Darryl Hany. PEvans and Theo acquired the licence to publish it around 1988 because they needed to have copies available for their popular postal version (and since GDW was around at that point I’m guessing it was licensed from the original publisher). The current edition, practically identical to the 1988 one, seems to be published by Paul Evans alone.
By the mid-eighties I think that the general tendency for players fancying a bit of X-number-of-Musketeers action was simply to play Flashing Blades instead.
Edit: James Maliszewski wrote a retrospective of the game, uh, rather longer ago than I recalled…