Why are major seventh chords called delta chords and written with a delta symbol, like CΔ7 or CΔ? Do engravers consistently use Δ to mean a seventh chord (such that Δ7 is redundant) or do some use Δ simply to mean major?
The triangle symbol Δ originally meant "triad" (meaning major triad) . However, nowadays it is - at least to my knowledge - exclusively used to denote a major seventh chord, even though it is a bit sloppy. I recommend you use Δ7 for denoting a major seventh chord. This will avoid any possible confusion, and it is also the symbol I come across most often.
 The Chord Scale Theory & Jazz Harmony, B. Nettles and R. Graf
△ without further qualifications in most contexts means "some type of major chord".
The point of these chord charts is to indicate to the player the most important information about the harmony of a given song. Usually, where the symbol "△" is used, we're talking about a jazz context. In this context, which specific major chord to use is left to the tastes of the performer, and so when one sees C△ in a lead sheet it might be rendered variously as C△7, C△9, C6/9 etc.
In the same way we often don't feel the need to tell the player which inversion to use, in jazz we often don't feel the need to tell the player which specific type of major chord to use.
Similarly, we often don't specific inversions; take for example a dominant chord (instead of a major chord), most of the time we see G7 any inversion of will do, but sometimes we really want the F at the bottom, in which case one would write G/F.
The same goes for major chords; if it's really important then be specific (e.g. C△9/G), and if it isn't then let the player decide what specific chord and inversion to use and just write C△