The mysterious absences of security seem arranged, to me, but arranged by whom? Is it Tabini, trying to give his grandmother opportunity to act or not act, the way Banichi deliberately gave Cenedi the chance to kill him?

I like this train of thought! Suspiciously likely. (Although maybe it's just that Bren's security is having more trouble than anticipated, given the antiquity restrictions of Malguri... Having to fetch parts, for example, or set up things manually that they normally would have subordinates do?)

Alternately, maybe they've been arranged by Ilisidi's security: testing what they're up against, if they decide to make a move against Bren?

Oh, of course. It's got to be Ilisidi's security; they've already decided to make a move against Bren, but haven't yet decided what move or when.