As described in the v1.36 release notes, sending a "synthetic" event to a contraption instance would cause it to be handled by the contraption's "external" (per-instance) script.
The deck-level script, like most scripts, is not executed while Decker is not in Interact mode. The only opportunity for a Contraption's scripts to execute during editing is the code inside custom attribute handlers, which is capped within a very brief quota. In both cases, these constraints exist to prevent incomplete or malformed scripts from wedging Decker and rendering it unusable.