The thing is, I don't believe assassination is the Magent's core activity. Being an agent is, which includes recon, intelligence gathering, stealing, sabotage... and assassination. One of the class lists deals with assassination (and can be used for anything else, really - a study patsy / create evidence combo can work for a theft as well as for an assassination), but the others are not specific.
From that, I infer that the Magent spells should be non-specific in their intent except for when the technical aspects are obviously specific - such as Open Ambush, Targeting and Well-Aimed Attack, which are combat-enhancing spells and, as such, specifically deal with combat activities.
Shadow Assassin is not explicit in which actions it should cover by its description, so the intent is that it covers any "deed", regardless of what it is - if the Magent wants to cast this spell constantly to escape divination when going about his daily routine and has power points for that, they are welcome to it. However, since a blanket scrying block spell without implicit or explicit duration would obviously be overpowered, we have to
- either assume the duration is indeed "varies" and the issue is with the scope, in which case we have to update the description to indicate that it covers one "deed" and only covers the actions that are directly linked to that "deed". The result is a spell that is difficult to ascertain and very much open to conflicting interpretations, but probably fairly true to the original intent of the author.
- or assume that it can cover any activities, but then, must be limited in time. I then consider the other spells on the list, which are mostly 10 min/Level (with a few being 1 hour/Level) and infer that 10 min/Level would probably be a fair duration. The spell becomes much easier to referee, even if it is probably a bit more removed from the original intent.
But I would not lock the Magent into an "assassin only" role just because of one poorly chosen word in a single spell description where it's actual role is being an agent that is really difficult to uncover.
As for the Dabbler, its main role is that of a burglar and con man - probably much more on the wrong side of the law than the Magent, and probably much less liable to leave corpses behind them as well. But I can agree with you one one point: the Dabbler lists are sometimes underwhelming and could stand to be improved in a few ways, notably in the ability to remove physical evidence of his crimes.